PC known (Correct me if i'm wrong someone) is known to the pound chat. You aren't bringing up a P/R/D/K out of no where that the pound chat has never seen, or wouldn't get suspcious about.
PC'ers used to like to call people out, if they knew X pet has always been on an inactive account, and then all of a sudden you have it.
If you had a PC known pet, and look decent, you know it's more legit and looked like a real trade.