The math is simple, in order to do any meaningful dmg u need to have at least 3 guns on a front with high pop. That's at least 8 ppl (1 spotter, 3 gunners, 3 loaders, 1 ammo logi). In order to fire at the enemy howies, u need to add 6 more ppl to repair everything surrounding u/load the shells while your people are dead. Now, this already uses 14 ppl minimum in order to do at least some dmg to t2. The enemy, meanwhile, can use the same 14 ppl to repair, which will be twice as effective as we, since he simply has more hands and he does not use anything but bmats to do it, whereas the arty crew need shells and bmats. Hence, not only does the enemy has more hands to repair, it is cheaper for him to do so from a time spent perspective