No idea why some bikes are faster than others, but on my normal 20 mile commute home, riding my '85 SR500, I'm usually about 3 - 5 minutes faster than on any other bike I own. This is a route I've timed myself on many times on several different bikes. It very well could be placebo effect, selection bias or pixies in the drive train.

That said, on a longer ride like 60 miles or more I suspect I'd be faster on one of my other bikes. I always feel a little more wrung out than usual after a ride on the Cannondale.