I think it IS based on kills as well. Same way as I explained that the fighter ranks worked only now there is also the ground damage, just making it more individual skills you are ranked by (10, as opposed to fighter ranks 5.)
On difference in the A/A part of the attack skill however is that the hit% is counted based on hits on both air AND ground targets, hence it's usually higer number than the players fighter hit%. (It's eaiser to hit a hangar with your guns than a manouvering plane).