Eureka! It's makes sense. I just didn't really think about it.
In the words "unique" and "usual", the "u" is treated as a y...a consonant (yoo-nique and yoo-sual.) So we just say "a" before them because...well, I don't know why we do it. Because we think the U is a consonant? Maybe it just sounds right to us. Or maybe English is such a messed up, confusing language that nobody knows why.