Something wrong with aircraft combat

This has happened multiple times now in the game I'm in. My interceptors are getting worked by my enemies aircraft, even when I outnumber him or his planes are on the ground (airfield). My enemies has 1 lvl 6 tac. bomber. Attack and def vs aircraft is 3.5 and 20 HP. My 2 lvl 5 interceptors with 10 attack and def and 20 HP are sent to attack it while it is sitting on the airfield. All aircraft have 100% condition. The result is that I lose 1 interceptor and his tac. bomber loses about 20% condition. Second time: I send 6 interceptors to engage 1 patrolling tac. bomber. Result is I lose 2 interceptors and the tac. bomber loses about 40% condition. None of this makes any sense to me.