No low luck will sometimes hit, if you only use 1 bomber. If you send over 6 bombers low luck will kill 1 every time as i understand it.
the 2 average loss is simple, 6 bomber runs should lose you 1 bomber so 12/6 = 2 average loss.
average damage is a little more difficult. a bomber deals 1d6 + 2 damage. now if you make 6 bombing runs your average roll should be 3.5+2 = 5.5. now if your shot down on the 6th try you get 5.5 x 5 = 27.5 damage. Dived by the number of bombing runs and you get 27.5/6 = 4.58.
Now if you look at low luck, you can run 6 bombers at 1 time, and you max out at 20 points of damage so your wasting some damage (and auto lose 1 bomber). That gives you 20 points of damage divided by 6 bombers = 20/6 = 3.333
However if you run 1 bomber per turn over for 6 turns, you may not get shot down at all or you may get shot down on the 1st or 2nd try.
Edit:
This is all without taking into account fighters, if you have 1 interceptor and 1 escort then things change more in favor of the defender because the interceptor hits on a 2 and the escort hits on a 1.
Over a 6 turn period you will kill 1 interceptor and probably lose 2 bombers (actually loss = 1 and 4/6th) to the interceptors. That means AA will only get to fire at 4 bombers so we can say it misses (4/6th).