I’m really pleased to see that your coin graded higher. I understand that TPGs rely more on the obverse condition than the reverse when setting the grade. Perhaps because the all important steps were least affected by the blemishes the reverse scored well enough to preserve the obverse grade. And I think you make a good point regarding the 70s and 80s being graded on a slightly different paradigm. I’ve had a difficult time establishing date/mm rolls in AU/BU condition from CRH events. Buying them may be the only option. I try to maintain the same standards and this rolls build slowly.
The practice of having different grading standards within a series is really not that uncommon, though it is typically isolated to strike or luster. For example, New Orleans mint Morgan Dollars are routinely given a pass for weak strike and flatness over the ear. If that same flatness existed on an early S mint Morgan, the strike would limit the grade despite the surfaces. Then consider Peace Dollars. The early common dates mostly have frosty luster, but some issues like the 1926 & dates from the 30’s typically have satin luster. That same satin luster on 1922-24 would most likely suppress the grade of the coin. That said, I still think this coin is overgraded. In my book, these coins should grade MS65 5FS.