Well, they ride a road bike 330 days/yr, if not more, so they are very used to the handling of that bike. What they are looking for is a bit of a mix between "as much as possible like my road bike" and "as good as possible for a TT bike". Going a little bit smaller helps with that, as the shorter frame will make it handle a bit more like the road bikes they are used to. And of course, they also have quite extreme positions so it is nice for them to have the shorter headtubes that go along with that.
Gerard Vroomen
3T.bike
OPEN cycle