Well for one the seat tube angles on tri bikes are so different, so that throws everything off what your prolly used to with a standard roadie.

Also, each tri bike manufacturers geometrys differ- alot ! I have people that don't fit well on the Felts, but the QRs fit like gloves and vice versa.
Lot's more measurements for even a basic tri bike fit are in order.

I'm on my way out the door, but do a search on bikesportmichigan.com. He's always a great reference. Also check out slowtwitch.com for some useful info. The forum scares me- lots of intensity on there, but you can get good info.