I've taken tests before and "known" every answer. The score didn't always turn out as I expected because I made silly mistakes by rushing. I'll bet the graders (there are generally 2 for every free response question though there can be 3 if there is argument as to the merit of a score, and it is scored in a blind fashion to what other graders see) knew the material better than you, and frankly you got the answers not right enough.
that you screwed up that day doesn't make the test invalid. It really just means you screwed up that day and didn't (I don't know you so this may not be true) perform at the top of your ability level.