They ride bikes with quite short chainstays for whatever reason. When you shorten the chainstay you also have to shorten the front center to keep the weight distribution from getting too rear biased. So short TT/long stem.

In this age of monocoque carbon fiber I'm not sure any of them actually have have custom frame geometry -- a custom mold for CF has an incredible price. Even Johan Vansummeren at 6'5" just looks to be on the largest, 60cm stock Felt with a 140mm stem and about three miles of seat post showing:




Of course Johan isn't a GC contender, maybe if another guy like Indurain comes around he'll get a custom frame. Then again Indurain himself rode a 59cm, albeit with another 140mm stem and a silly looking headtube and steerer extension: