I think I may code up a simulation tool just to test scenarios. Anyway, if we set up a situation with a high damage/low def class and a low damage/high def class, we can adjust defense and offense so in a 1 on 1, they both drop at the same time. If we expanded this to teams of high damage vs low damage, again everything is nice and equal, with one member of each team dropping at the same time.
However, if we look at the case of a pure high damage team 1 (say 4 APU) vs a mixed team 2 (say 2 APU 2 Tank). Both sides focus fire one APU from the other team. However, team 1 has more damage, so they take down team 2's APU first. Team 1's first APU falls shortly thereafter, but not before they land a few hits on Team 2 APU2. Team 2 APU 2 then drops next, taking damage from 3 enemy APUs these 3 APUs then begin on the tanks. The 2 tanks of team 2 do take down APU2 from team 1, and we're in a 2 tanks vs 2 APU situation - except one of the tanks is now down to half health ish. So at the end, team 1 wins. If you make the pure team 100% tank, you see the same result - the pure team has periods where they outnumber the mixed team, so they win.
I'm not sure what happens when you start adding in "in between" classes etc. But you get the picture.