LOL. Good Catch! I read that as "perpendicular" about a million times....don't ask me how! Funny how what you type and what you read can be 2 dif things!
Heh... very common mistake for writers (why we need editors... ).
Once the brain has written it down, and associated the "incorrect" word (or phrase) with what it know's was supposed to be written, it just automatically replaces it every time you look at it again.....