I have always been a bit of a critic of the BGS grading system, and grading overall actually. It just seems another way to suck money from your pocket to me. Anyway, I don't know why they just don't average it out to get a more accurate reading overall of the card.
Say you get 10/9.5/9.5/10
On average, your card is 9.75, yet with the BGS system, it is downgraded to 9.5? Makes no sense, and in my opinion, it would make things clearer for collectors. In order to get a perfect 10, then you should have to get 10/10/10/10. One 9.5 should bring your score down to 9.875. Still a score to be proud of.