Vertical "harmonization" is also set with convergence.
Rounds leave the muzzle at an elevation set to have the round drop to the gunsight center at the convergence distance set. Ballistics characteristics of each weapon are included in convergence setting, so MG and cannons would meet at the same elevation at the convergence distance if set the same.
So, setting your convergence out very far could cause you to shoot above a target that is very close to you, even though the gunsight is on the target.
Here is a link for more info:
Trainers site>>The netaces link in my signature has more info on ballistics also.