Every skinslot in your loadout haveing a number of coded skins, which will be used. M4A3(76)W was in AA campaign, that's why it has all skins from AA. Also, it has historical skins, which were created by players and added to the in-game store.
You can't just apply skin from one vehicle to another, because it's not working in that way, even when models have a lot in common.
Also, it's not the only issue with skins. Here is the list of all inconsistency, which i saw:
Soviet M4A2(76)W - didn't have skins untill 2015-2016
Ostheer SdKfz 234/2 "Puma" - didn't have skins untill 2015-2016
OKW Opel Blitz - doesn't have any skins at all. I suggested to give it panzergrau as default (to be in line with sWS, Tiger and SdKfz 221), but it didn't make it.
OKW Tiger - doesn't have any skins at all, using a panzergrau as default.
OKW SdKfz 221 - doesn't have any skins at all, using a panzergrau as default.
UKF M5 - doesn't have any skins at all.
UKF Achilles I (a.k.a. british M10) - doesn't have any skins at all.
We were trying to push Relic to cooperate with skinmakers, but they decided, that it will require to much work.