For some people the TT position reduces FTP; their muscles don't fire as effectively at the different extension/contraction points or they don't breathe as well. I actually put out my best 20 minute numbers on the TT bike, so it's a case by case thing. The climbing vs. road racing output also seems to be pretty individual; again this may be as much a position thing (hoods/tops vs. drops), though I've seen/heard of far fewer people having any significant variability when they are tested back to back between the two.