Armour penetration ratings

[Vartuoosi]

You have just been unlucky. Or shooting from a bad angle.
Even though the stats state that a gun can penetrate X amount of armour, the shell can occasionally be stopped by a lot less protection, depending on a number of things.
There are a lot of variables in play.
I've seen a T-34/76 bounce of a shot fired by Jagdtiger once but that was nearly 2km away of I remember correctly.
But I think the armour penetration stats are fairly reliably.
I've killed plenty of big cats with small guns.