When I have a battle, not all the available reserves show up in the reserve box - seemingly random numbers of units in that GHQ and other HQs show up - for instance, I have 5 german 564s in my GHQ and never more than one of them show as avalible to put into a battle
the last battle on the west front should have shown at least a dozen german units from the GHQ and other HQs, it instead showed 3 aircraft, 1 unit from the GHQ, and an austrian unit from the east front GHQ lol
It happens on attack and defense, and irregardless of the distance of the ghq to the battle hex. Also, the order in which I have units lined up in the GHQ doesnt matter either
Fortunately the Allied ai never fights battles more than a few rounds in attack or defense lol, but if they ever did I would run out of units fast