What we are REALLY saying...

The concept of choice as a representation of freedom is an illusion.  To prove this all I need to do is ask you to choose to be hungry...........now.  Its impossible.  Hunger is controlled by something primeval.  something that existed before you did, therefore you have no control over it.  Because if you did, with your limited abilities, you would muddle up the timing, the amount of food for satisfaction, etc.  you would kill yourself from simply not knowing how to control hunger, therefore you can not control it, and as such you can not choose to have it or not.  when you eat because of hunger, you are simply REACTING to it.  So as far as hunger is concerned, you never choose to eat, you only react to hunger telling you to. 

This logic can be applied to ANY desire we have as humans, and it just so happens that desires are the only thing we have.

everything we do is simply a reaction to a desire.  and like hunger, these desires existed long before we were.  as we live our lives and react to our environment, we simple discover them.  to prove this all I need to do is ask you to like something you don't like ...........now.  Ok I will be fair, You can dislike something you like, but you have to do it.......now.  Were you able to?  No of course not.  this means, like hunger or sleep, all our desires, even our most personal ones that we hold closest to our hearts, that we use to define who we are, that we use to express to people our uniqueness.....they all existed before we were and are controlled but something besides us. (please understand I use the word "us" so lightly, I mean who the hell are "us" anyway?)

You never chose to like blueberries.  You never decided rap music wasn't for you.  You only reacted to a desire that expressed itself when you happened to eat a blueberry, or listen to rap music.  And this desire isn't simply "I like rap music", these desires are not quantifiable in any way.  they are the meat and bone, the sweat and blood, the air and earth, they are everything, and it so happens that within your little puny organic structure, the slow beat, the deep bass, the hard voice of rap music satisfies one of them.  Or it doesn't.

Whatever the result, you didn't choose it.

so how does this relate to communication?

simple.  Since every choice we make is in actuality only a reaction to a desire, when you say something to someone, and they say "i understand" unless what you said is satisfactory to one of their desires, and that desire caused them to react with the words "I understand"  they do not understand what you MEAN, they only understand the words you spoke.

by speaking to someone you could say you are giving someone an opportunity to choose to understand what you say or not, but since there is no choice, only reactions to our desires, the only way someone will understand what you say is if what you say satisfies one of their desires and causes them to react with the words "i understand".  any other time the words "i understand" are being spoken they are empty, because the words you spoke are not satisfactory. 

Even when you speak words that are satisfactory to someone's desires, and the desire causes the person to say "i understand",  the desire within you that caused you to speak could be different from that person's desire that caused them to say "I understand" and there just happened to be enough in what you said that satisfied them.  therefore, because of this discrepancy, there is never a time where you can safely say that someone understands you perfectly.

The only way this discrepancy could be weaken is by SEEING actions, over an indeterminate period of time, that satisfy the desire that caused you to speak, from the person who confesses to understand.

Because of all these discrepancies: the intricate differences between desires that make up each individual, AND the ignorant way in which we use language, It is near impossible to ever be certain anyone understands you at any point in your life.  Good luck.