This is a possible line of reasoning for the conditions you described.

The lower weight of an A-class means less hull resistance (in general) and lower righting moment (in general).

Lower resistance directly influences speed at all points of sail.

Lower weight may also reduce their pitch radius of gyration (how easy/hard it is to rotate the boat in pitching) which has been shown to improve the performance of boats, particularly upwind.

Lower righting moment means that they can fly a hull with less pressure in the rig, which means they don't need as much draft in the sail (the sail is flatter in general), flatter sails have less drag and can also point higher. Both increase VMG.

A-classes also have less "stuff" dangling about in the wind creating drag. Items like spinnaker poles, chutes, bigger hulls and extra crew all increase the windage (drag from wind) of the boat, which is slower.

A-classes are lean, finely tuned, upwind dominating machines. In their condition, there is not much on the water that can touch them.