You have to look at it this way Lainer, it's 81mm HE projectile hitting the 10-30mm top armor of a tank. It might not penetrate but it will probably cause the inside top armor to spall to hell, basically like a grenade going off inside a tiny medal room, just without the pressure wave. This is true especially since they probably didn't line the top of the turret with spall liners.
So in real life its likely possible, just unlikely to happen more then a couple of rare occasions, and in the end is pretty much entirely luck.
In game, if you make contact with the tank directly with a mortar, you can often 1 shot it. Very close hits will often still do a bar or 2 of damage as well, like artillery rounds. Quervo 9the mortar fanatic) did it a few times in campaign 20 and a couple times this campaign over at WaW.
The AVRE in game will actually kill 2 Panthers or FOUR PIVHs if they are grouped together within the splash radius. The Tiger can also kill Greyhounds and Pumas with HE. So HE can definitely kill tanks with direct shots, depending on the size of the round.