Why is this a BGS 8 and not 8.5?

Just got a sub back and got the following subgrades of 9 centering, 9 surface, 9.5 edges and 7.5 corners. A real gut punch on that last one…
Overall grade was an 8

Earlier last year I had a card with subgrades of 9 centering, 7.5 surface, 9 edges and 9.5 corners.
Overall grade was an 8.5

Is there a reason why BGS did not scale up the same in both cases? I thought grading with subs meant absolute clarity in a grade or is there unseen values afoot?

I believe I found the answer to my question here.

Basically, BGS values corners more than surface, which is a bit surprising. So a 7.5 weighs down a score more for corners than it does for surface. Hence the difference in grade for my card.

Win some and lose some I guess?