I do not know the number cruching mechanisms in the game engine, but my common sense says this: long range, (somewhat) inefficient guns, HULL DOWN positions and lots of shells spent should generate lots of gun damage, because most of the succesfull shots will hit the turret where the main gun is mounted (in the most cases).
So, could it be the case, that variances in gun damage percentage claimed by different players were altered by scenarios played (suitable hull down positions, equipment used) and skill level of the players (ability to find good hull down positions). Yet I find it hard to believe in 80%+ percentages. Ofcourse really high percentages can't be explained by this, but maybe a moderate increase in gun damage.
Well, my 2 cents