Isn't the 9.5 Mint+ by intention a subjective grade to designate a card that has a gem mint appearance to the naked eye?
(this is my understanding, I don't know for sure).
If that's the case, then I could see a definite weighting being used in regards to the subs. Edge grade would be nearly inconsequential between 8-10 grade, most people can't see it at all or the difference between small increments at high grade (especially in a slab). Centering would be weighted the highest, it's the easiest to spot a clear issue, even at 55/45 to spec for gem it can sometimes noticed on the right design. Corners would be the second highest again as an obvious or apparent defect if lower grade.
Surface is hit or miss. If it's a 9 surface grade the mark or imperfection could be on the back, the issue could be with coloring, it could be dinged under a difficult to spot area (white, text, business on the design).
If something like the above is correct, perhaps the key for 9.5+ is 9.5 or better on centering and corners and more lenience on the edge/surface. That would align with your overall 9, since corners have a possible perceivable imperfection.