No, I was only joking!! you actually have a valid point, although every time i have had someone in front of me it has been a road bike, and others I see on a mountain bike I either keep the pace or overtake, I also don't just cycle to work, I also train with weights 3 times a week too, So I am reasonably fit.
I think on a sprint if we were both on the same bike, i would be slightly embarrassed if this heavy lad whipped me!!!
As was said 2mph faster would have been a reasonable figure that he passed me on a hill.