I fly the 190a5 often too.
Consider:
1) YOu are sitting HIGH on a tall beast.
2) Cannon have lobbed trajectory. 20mm aren't so bad, but 30mm are. For the A5 there's 20mms, and you have to consider that they are aimed IN and they are aimed UP so that the round will shoot OUT, UP, reach an apex then fall DOWN to land at the bullseye for your convergence.
However, they are much lower than you, as they begin on the wings themselves. Personally I find convergences unrealistic. There's no way you could set gun convergence to 50 or 150. The guns would be 45 degrees inward, and they had only so much room for adjusting. HOwever, suspending disbelief, imagine that they start at the gun barrels on a straight lin to the bullseye. Even with that assumption (which is false) the bullseye is aimed to the cockpit. So that means at any given range the guns shoot UP. So the closer you get the lower the rounds, because you're cutting them off before they reach full height.
Think of a right triangle. It's got a fixed height and width. If you follow the line the full way it will reach its full height. However, if you stop halfway down the line it will be much lower than the full length allows.
So hitting targets super close does the same thing. Compensate if you're THAT close. Mostly however i've not had too much trouble with that.