Crank length wouldn't matter here, that doesn't affect gearing, just effort level and whatnot. Tire pressure and small variations in tire size among manufacturers wouldn't give you a 20% or more difference.

What is most likely going on if you're actually in the same gear is that his pedal stroke is rounder and cleaner, so he's actually pedaling all the way around at a constant speed while you're coasting slightly at the dead zones (cranks vertical). This is easy to do if you're really hammering/grinding it out. I'd think you'd hear it given that mavic freehubs are pretty loud, but maybe not.