Some of you made a very good question on why the RU 251 and the Kanonenjagdpanzer have the same gun but their statistics are different.
I got on my investigating feet and asked around.
Basically in real life the RU 251 and Kanonenjagdpanzer have the same gun and same ammo but in-game they have same gun but different ammo (AP).
Also the guns are technically different as its impossible to have the same module available as a standard and premium (research/grind reasons).
Hope this satisfied your curiosity and very good observation as always.