About twelve years ago I got a PSA 6 on a card that I was sure was going to get an 8 or 9. Under magnification the corners are sharp and come to a point with no hint of wear or flaking. I was upset and had it reviewed at a show and they said there was an extremely minor wrinkle that accounted for the downgrade. All I could see under 10x loupe at the point indicated was what looked like a natural bump in the paper mulch. A year or two later I re-submitted it raw and got the same response. The PSA guy at the show said ex/mt 6 was the highest grade a card could get with a very slight wrinkle. They were certainly consistent.
JimB