I don't see biennial as presenting a spelling problem at all. It doesn't make sense to me why it was included as a spelling problem.

biennial:

It looks right.

bienial:

That shore doesn't look right--that would look like something pronounced: bie-EEN-yul

biannual:

That just sounds altogether different. Of course, if you stare at any word long enough, it starts to appear to have been incorrectly spelled. Wonder what's the psychological term for staring at a word until it begins to appear to have been misspelled?

But biennial probably presents more a problem of being used incorrectly with regards to its definition that to its spelling.