RNG fiddling with gun depression?

DomoSapien: This isn't an RNG element, moreso a biproduct of how shell trajectory is calculated in World of Tanks. At least, that's what it sounds like based on your description   As you probably have noticed at some point or another, large caliber HE rounds fired out of a Derp Gun or Howitzer will actually arc upwards ever-so-slightly then drop down in time to hit their target.

That is because the distance the shell needs to travel is calculated based on your aimpoint. In trying to account for a physical target and calculating the shell trajectory for a shot originating at the base of your turret, there is a bit of a 'jump' with the reticle when it's presented with a physical target over a ridgeline at a considerable distance away. 

Anyway, this is my understanding of this phenomenon. Hard to say without a replay.