Posted By:
Eric BrehmJosh -- I think an EX 5 card can have a very slight wrinkle. From the grading scale published on SGC's web site:
"60 EX 5: 80/20 or better centering, minor rounding or fuzzing of corners, roughness or chipping along edge (no layering), one VERY slight surface or "spider" crease may exist on one side of the card, gloss may be lost from surface with some scratching that does not detract from the aesthetics of the card."
PSA is not entirely clear on the matter -- their definition of VG-EX 4 says "a light crease may be visible," but their definition of EX 5 doesn't mention creases or wrinkles at all. I would take this to mean that creases and wrinkles are not allowed for EX 5, but as you say, PSA 5's do in fact sometimes have them.
Anyhow, when I see a PSA 5 that looks at first glance to be higher grade (sharp corners), I usually suspect that there is a small wrinkle somewhere that is difficult to detect.