Actually, there are some cases where AT gun shells clip through the ground and destroy their target. Steppes, for example when there is a tank at mid fuel on the hill and there is an AT gun at the bottom. It can shoot it, at least i randomly lose tanks to weird physics.
I'll explain this with an edit of another post I made once:
It's a side-effect of how ballistic weapons (all tank guns and AT weapons) work, there's no way to fix it. This goes for all AT guns, AT Rockets, Tank Guns, etc.
There are 2 systems in place, the first one works like Accuracy × Target Size = Chance of guaranteed hit. If a hit is guaranteed, the second system does not need to come into play, and the shot will hit nomatter what even if it has to phase through walls or turn in mid-air.
The second system is Scatter, in the event the shot is not a guaranteed hit, it may still hit depending on the vehicle's scatter. On the other hand, scattered shots can hit terrain and objects. So it can hit, but it's not guaranteed, and on uneven elevation the chance to not hit is larger.