Proofs are held to a higher standard and rightfully so. That being said, early proofs often get a bit of leeway because they were routinely handled, cleaned, and wiped down.
I think ANACS net graded this coin, which is in line with how Proofs have historically been graded. In other words, ANACS likely started at a base grade (PR65) and then deducted grade points for hairlines in the fields and impaired luster, and arrived at their conclusion of PR61. My experience with early Proofs is that since strike quality, wear and circulation are not usually relevant factors, the differences between PR60 and PR65 are instead accounted for by eye appeal. A Proof 60 coin and a Proof 65 coin might have the precise same strike quality (especially if struck from one die pair and in limited quantity) but whereas a PR65 coin looks nearly untouched, a PR60 coin will show extensive signs of handling, distracting marks, poor luster, etc.
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