It's because it's a two ply head, it doesn't happen to single ply heads. They're not going to lay perfectly flat/together when they are put on the collar. Apparently there's a bit of a problem with Remo regarding this, not sure if Evans has the problem as well. But if the two plies aren't manufactured properly it can cause a "dead head", which is why you occasionally see people in the drum section holding up heads by the collar and lightly tapping them close to their ear. The theory is if you tap it and it puts out a slight tone, the head is good. If it doesn't, and it sounds papery or plastic in a sense, the heads aren't going to open up as well as they should. I've tried it and never noticed a bad head, but then again maybe I've been lucky. Some cats swear by it.