I think this is likely true for a couple of reasons. Especially though, true though for people who played private servers 
I didn’t try it out til after classic came out.
Some stuff they have is just flat out more vanilla if that’s even a good way to say it
Isn’t that why there were private servers?
Well, Classic just shows that even Blizzard cannot recreate Vanilla perfectly. PServers had some things right that Classic got wrong and vise versa.
Despite what some in this thread may think, Classic is not Vanilla and is completely different game code and client so this just reinforces my idea of a PTR for Classic. They are using a different code base from Vanilla and no matter how much they “Think” they recreated it right, when new content is being released, it could be good to send it to a PTR 1-2 weeks early so the players can also try it out to make sure it is working the same.