What interests me here is the distinction between lies and BS. With lies, you are against the truth. With BS, you just don't care.
So it has a different purpose... or may not!
You may be giving BS not to convince others of your words, but to convince them of something else: say, you give 'em a lot of crap detail about issue X, not because you care what they think of issue X, but to convince them you're smart. You may want to distract them from something else (like why you haven't done something you promised). You may want to make sharp, clear questions seem fuzzy, so you can get away with something. Or not face something.
On tests, you may simply be hoping some of your guesses happen to be right. So you make lots of them.
It reminds me of the poetry of T. S. Eliot. He's not talking about mermaids and peaches and Michelangelo; he's talking about feeling old and insecure... but it works.
Those are my random thoughts on BS. What's yours? Do you BS others -- or yourself? Is your BS detector online -- and do you find others' to work too? (Feel free to BS, but be warned: with a thread title like this, everybody's going to be on the lookout for it!)