A bisexual man I know explained it to me as this:
Bisexual means two-- you like biological men and women.
Pansexual means a gamut--you like men, women, and anything in between. I.e., androgynous, transgendered, intersex, etc etc.
So he says he identified as bisexual and not pansexual because he "likes his men very much manly and his women very much womanly". (His words, not mine.)
So that's what I've been assuming since.