How they get a final Grade 0

[416LANCE]

I have sent in over 150 cards so far to CSG and have been overall pretty happy with the results, understanding the scale differs from PSA and Beckett. That said I can't figure out how they get the final grades for instance I have had most of my cards with subgrades, so I have a 1984 Donruss Don Mattingly RC, got a 7 centering, 9 Corners, 9 edges, 9 Surface and a final grade of 7.5, not sure how they got to that? or a 1984 Dan Marino RC that got 8 Centering, 9 Corners, 9 Edges and 4.5 Surface and got a 5.5 final grade and a 1960 Carl Yastremski that got a 8 Centering, 6.5 Corner, 7 Edges, 5 Surface and got a 5 Overall, I just don't understand the math, can any explain the breakdown?

[jsrva]

Generally, a final grade can be no higher than 0.5 above lowest sub-grade. However, I think there are some exceptions to go one full higher grade above lowest sub. I have no explanation for your '60 Yastremski.