That's a bit of a head-scratcher (i.e. looks good enough for a 64). Which generally means there's something wrong with the coin that's not showing up in the photo, rather than a mistake by PCGS.
This one has me scratching my head also. Picture is a real good representation of what the coin looks like in hand. Like you said probably something I am missing. Time to renew my membership with the collectors society so I get the 5 early bird grading coupons from NGC. Might just send this one in. About a $275 difference from ms63-ms64 in this grade so might be worth a shot.
I think this is one of those cases like we were discussing the other day. The coin is one of those that is actually AU but the wear is so slight and the coin so clean that PCGS bumped it to MS63.