Transports power differs

I have two transport groups sailing. One has a cruiser, 2 destroyers and 80 armies. The other has a bunch of planes.

This is pan asian lvl 4 transports hitting ships at a 3. The cruiser hits at a 5.5 and destroyers at 2.5. So the group with cruiser would hit back with a couple more points then the airplane group, right.

Yet the battle screen shows group with cruiser hitting with a 21 vs the planes 29.

And the cruiser group is healthy. 2020 hit points out of 2077. I don't get it. That's one question.

Another is since the planes never merge, if that group is attacked will they be a separate group, like only the group attacked would return fire?