Is it me? or does it seem that low value zero range AA fire tends to shoot down much more air units than ranged high value flak batteries and battalions?
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.
In the Fall Kreml campaign I'm playing, AA losses seem incredibly high. In German air attacks against fortified infantry positions, I lose a plane every second or third time that I attack. Seems far too high, if infantry battalions were this effective in shooting down planes there wouldn't be much point in AA batteries or any defensive air efforts.