why the gunnery is so much differnce between il2 and CloD?

[Danelov]

That depend also of the damage model and the number of parts of the model. I dont know how that works with Clod but as example, with CFS2 each part had a number of points in the DP file: engine for example can be 390, coolant reservoir 195, oil reservoir 205, aileron control cable 50, etc,etc,etc.
After the part is hit and the points are consumed, the part is considered as damaged or destroyed. With all points out of one vital part the plane go down. That work conected with the value of each bullet or shells from the weapons; as example the 0.303 bullet had a valour usually betweenn 7 or 8. A 20mm shell valour is 36, etc, etc.
Of course, that is CFS series, another world.