i think it's because, on the average, fixie riders attack hills more aggressively than do roadies. "average" is key here. there are fewer fixed riders in the world than there are roadies, and most of them are riding fixed gears on purpose and have some sort of dedication to the idea, so they are more likely, "on average", to put a certain type of effort into climbing; they have singled out climbing, in their minds, as worthy of special attention. on a geared bike you can shift down and approach hills in a low-impact fashion and it's not a big deal, so the "average" roadie doesn't attack hills with the same ferocity of the "average" fixed rider.

of course, there is great variation among individual riders, as ken has documented. that's what makes it interesting. i wish there were more girls to race around where i live. the other day i passed a road rider who immediately got onto my wheel and stayed there. i kept my usual speed up a long, gentle climb and then figured, okay, i'll let him pull for a while, so i drifted to the left and he stood right up and mashed away as hard as he could. such a gentleman! of course i couldn't stay with him - until the next uphill grade, of course, where i caught him easily. then he had to make a right turn onto a dirt road for some reason. *sighs* ann arbor velo club...