damage dealt incorrect?

I had 51 interceptors attacking another stack of 76 planes, of which 26 were interceptors. Mine were lvl 7, enemy had lvl 6 interceptors with missing health. My dmg value was 325, his defense value was 115. I attack, and somehow I lose 270 hp? Is there some feature I'm missing out on, or was this a bug? I used 10 aircraft I split off from my main force for my first attack, and I tried it 2 more times, and each time I lose roughly 250 hp from my stack.