RE: AA results
If by low value zero range AA fire you mostly mean: infantry battalions, it's because the AA fire value of an infantry battalion is reasonably decent compared to that of an AA unit.
A 800-900 men battalion has a similar AA value to a 12 gun AA unit with an AA value of 15. The fire value isn't displayed with on map results off, but interpret an 800-900 men unit as a 80-90 guns with a value of 2, divide that by about 8 to get to a value of about 15-16 to compare that to the AA unit and the result is similar.
Plane losses, like gun and vehicle losses are also a bit of a lottery as they're based on a percentage chance based on losses in men equivalent (10 men=plane/gun/vehicle kill): AA units won't be able to shoot down 3 planes from one roll, but 3 infantry battalions stacked together might get lucky and shoot down 3 planes.