You're missing the fact that after A1's attack, A2 is facing a weakened D1.
There is no kind of reasonable assessment that a military force in 1914 would have 85% of their rifles/guns pointed in only one direction, and would thus be unable to shoot at an enemy approaching from the rear. In a stack of 50 units, 30 of them aren't fully invested in the "frontal" attack, so they're necessarily able to defend against attacks from the sides and rear. Even in a stack of 20: 20000 men don't all shoot in the same direction at the same time.
The percentage loss could more reasonably be up to 15%, maybe, maybe, but I believe the damage from previous combat already accounts for that.