I can tell you that my friend and I have done this test several times and it always comes out the same. I weight 163 lbs. and he is 115 lbs.
We climb up this hill all the time and come back down its about 1.5 mile downhill all the way down with a grade range of 3 – 10% without pedaling we start out at about 15 mph and coast all the way and I’ll hit 39 – 40 mph he gets up to about 37 – 38 mph and I’ll get to the bottom about 5 – 10 seconds before him.
He says that as soon as we start I’m pulling away and he never gains any distance the entire way down I just leave him.
Why who knows you tell me but the results are always the same.