How is the final grade in BGS calculated?

Would appreciate anyone's input with the knowledge of how Beckett comes up with the final grade for a card. I always assumed it added the 4 sub grades and then divided that by 4 to reach the final grade. e.g. sub grades of 10, 9, 8, 9 would be a final grade of 9. Is this correct?
I'm asking because I've seen a card with grades of 9.5, 8.5, 8.5, 10 which by my calculations is a total score of 36.5 making it a BGS 9 yet it's a BGS 8.5. Some help in understanding would be much appreciated.

I copied this from Beckett,

How Final Grades are Determined
Here is how we do it!
The overall numerical grade is not a simple average of the four report card grades. Beckett Grading Services uses an algorithm which determines the final grade using the 4 sub grades on the front label of the card holder. The lowest overall grade is the first category to observe because it is the most obvious defect, and the lowest grade is the most heavily weighted in determining the overall grade.
Example:

• Centering = 9.5
• Corners = 9.5
• Edges = 9
• Surface = 8
• Final grade = 8.5

The reason that this card received an 8.5 is that even though the Surface grade was an 8 (the lowest grade overall), the 9.5 grades on Centering and Corners were strong enough to bring it up a full point to reach the 8.5 level.
Another example:

• Centering = 9.5
• Corners = 9.5
• Edges = 8.5
• Surface = 9
• Final grade = 9

Upon first glance, it may appear that this card should've received a grade different than a 9. The most this card could receive was .5 (or one-half grade) above the lowest sub-grade. The Edges were the lowest in this case, hence, the card received the overall 9 grade. Even though Centering and Corners received grades of 9.5, a key point to remember is that the minimum requirement to receive a grade of Gem Mint is to have at least three grades of 9.5 and the fourth to be no less than a 9.
Also, please note that the final grade rarely, if ever, exceeds two levels above the lowest of the four characteristic grades. For example, if a card has characteristic grades of Centering 10, Corners 6, Edges 10 and Surface 10, the final grade will be a "7" (of which is exactly two grading levels above the lowest characteristic grade).

Thanks Ahmad.