I think the overcast conditions allows more moisture in the air, thus making the wind heavier to be able to move the ball more than light dry air. V = M x A coming into effect?
NB: Just thought, it could be; no lab test conducted by me or no ref.
Rough surfaces in the old ball is much better used for reverse swing, when quick through the lighter air (Sunny) but bowlers who generate good pace could reverse in almost every condition.