If you use the Battle Calculator and try some simple combinations you will find something unusual. There is almost always a range of attacker/defender combinations that yields a 0/0 or mutual destruction result.
Fighters vs FightersEdit
First try 1000 attacking fighters versus 1000 defending fighters. Not suprisingly the result is 0/0, or mutual distruction. Now try 999 verses 1000 defenders. Result is 0/1, the defender has 1 survivor. If you reverse it 1000 vs 999 and the attacker has 1 survivor. Seems reasonable.
Bombers vs FightersEdit
Now try bombers against fighters. Bombers have twice the power, so it is no surprise that 500 BM vs 1000 FT = 0/0.
However 501 BM vs 1000 FT = 0/0 and even 510 BM vs 1000FT = 0/0. Very strange!
Indeed even 1000 BM vs 1000 FT = 0/0 !!!
You have to have 1001 BM vs 1000 FT to have a single Bomber survivor.
Why The Window?Edit
Why? Well Bombers have Power = 4, but their Armour is only 2 (same as fighters), and so they don't begin to survive until their armour is equal to the defenders power.
You have a Mutual Destruction range, or Window, of 500 Bombers!
This is the reason attacking Fighters with Bombers usually gives a poor ratio. The best you can accomplish is to break even with 5000cr of Bombers killing 5000cr of Fighters. But in the worst case it costs you 10,000cr of Bombers to kill 5000cr of Fighters!
In almost every attack situation there is a Mutual Destruction Window, and that means that you can literally waste a large number of units and gain nothing. The trick is to determine how few attackers will do the job, any more just fly out that window.