There is the possiblity that the electric contact in the sockets improves slighly after a few hot cold cycles, (after which it begins to degrade due to corrosion) which could account for a peak of performance. My guess is that this effect would be so minor as to be unnoticable.
However some components, may be just a little better then others, perhaps in a perfect world, both boards could reach the higher speed, but in this case the slow one is not quite as good.