The short answer is because what you are saying can be very different from what your audience hears.
The longer answer is because what you say is hardly ever all that you mean – no matter how honest you think you are.
For example, what if you are at work, and you want your boss to get off your back and stop hounding you about something? Although there are some cases in which you might go and directly say something, this is often not the best course of action. Usually, if you say anything at all, it will be addressed more gently, not revealing your annoyance. Often, you won’t say anything at all.
I also notice this phenomenon in the area of dating and romance: people are so concerned about how they will come across that they modify their expression in order to give themselves a better chance at being heard. After all, it would be nice if you could just come out there and say that you want to rip somebody’s clothes off and ravage their body and soul with unbelievable sex – or maybe that you want a lifetime relationship, in which you have a house together and 2 or 3 kids, and that you’re just looking for somebody who might be willing to take that plunge with you! Wouldn’t that be great?
Unfortunately, this is asking an awful lot of anybody to whom your request is posed. Folks who might be open to these scenarios later on will often run the other way if you throw it at them without a contextual history and background within which to understand it.
And so it is that in situations of potentially higher drama, when getting to the truth is crucially important, often this is, ironically, when it becomes more difficult to “say what you mean and mean what you say.” Because the stakes involved are not just about the answer to a lingering question – they also often involve unintended value judgments and hidden conclusions about what people are thinking (and even how much respect they deserve). The boss whose employee asks for some freedom from constant surveillance might be left thinking that their employee does not respect them or their authority – not the intended consequence at all, but these side effects often happen. Or the boss might take it personally and conclude that their leadership skills as a boss are faulty. Again – unintended collateral drama.
In dating and other social relationships, this drama often multiplies upon itself. For example, Person A doesn’t tell Person B the whole truth about how they feel about one of Person B’s persistent habits, and a tension develops. One or both people notice the tension on some level, even if not consciously, and it interferes with their ability to get along – but nobody says anything until finally, one day, something breaks the insecurities wide open and a huge flood of drama that has been building up comes pouring out, and people get hurt. This underscores the need for us to have clear communication skills, to avoid unnecessary buildups and dramatic blowups that can occur when straightforward communication is not happening.
Another problem that often develops is when somebody is used to not being able to share their whole truth. They’ve had experience after experience that confirms to them that when they straightforwardly share exactly what’s on their mind, their open sharing inspires a shitstorm of everything from mistrust to confusion to unnecessary, unsolicited judgment about who they are as a person! This especially happens to outsiders, minorities, and people who are more subject to scrutiny for who they are in general.
When somebody experiences such constant misunderstanding and judgment, they often learn to think about what the best thing to say is, as opposed to the truest thing. It could be part of the truth, it could be a simple lie, or it could be something that shifts the focus or attempts to change the subject. Which is why we need to stop seeing “say what you mean and mean what you say” as being only the sharer’s responsibility. Because, from the sharer’s viewpoint, the question is “what is the best way to get you to understand me as best as possible?” If you truly want to hear that, you have to be willing to listen to them and work together to get to that place in those moments when the communication is not so easy and automatic.
One of my best friends and I have constant misunderstandings. At times, we have literally spent hours untangling our misunderstandings – because sometimes one doesn’t know what the heck the other is trying to say! But the strength of our friendship has been built on the desire both of us have to understand the other. Both of us can actually be really direct, honest, blunt people – but that doesn’t stop us from getting misunderstood! :-)
One more thing: I know of some people who think honesty is easy – just tell the truth, the whole truth, and nothing but the truth. Ok – nothing but the truth is not so hard to figure out, I’ll give you that. But if you want to talk about the whole truth, well … nobody ever actually tells “the whole truth.” That’s because this is impossible to reliably do, especially if someone asks you an open-ended question. If you are sitting in your living room and somebody texts/emails you “what’s up?” and you answer “I’m sitting in my living room,” you are telling the truth – this is not a lie – but you aren’t telling the whole truth. You might also be thinking about things while sitting there, or maybe eating a bowl of oatmeal. Or maybe you’re waiting for the massage therapist to stop by for your massage appointment. Or waiting for your kid to come home from school. Or perhaps you’re sitting there because you’re tired, and maybe you’re watching TV. But, you see, you didn’t say any of this. You didn’t tell “the whole truth.”
I often get this kind of weird confusion when people ask me where I’m from. Sometimes I humorously reply “from my mother’s womb, of course.” This isn’t a lie, I did indeed come from there. :-) But it illustrates (1) how the perfectly valid truth that one person speaks can [often completely by accident] not line up with the truth that another person is looking for, and (2) how liars, cheaters, and other dishonest folks can often say things that are perfectly true and still continue to lie, cheat, and deceive.
The biggest myth about honesty is that it is a purely passive thing (i.e., don’t fabricate lies). That’s only half the battle. The most honest people go a step further than just not lying: they see their honesty through from start to finish. They verify, as much as possible, that they and the other person are on the same page about what they have communicated. People who bluntly say shit and claim “I’m just being honest” all the while knowing that their “honesty” hasn’t encouraged any real understanding are practicing lazy honesty – which won’t win you any awards from me.