The main difference is that PSA defines what their centering requirements are for a grade, while SGC does not. So yes, a 60/40 card could still be a 10 if every other characteristic is flawless, according to their definition. Normally they will give it a 9.
Would the OP gripe if it was a 9(OC) rather than the 7, but this would probably get a 6 from PSA if no qualifiers was selected. BGS is the toughest of all three on centering, although this got a BGS 7.5 centering subgrade:
1992 Topps - Stadium of Stars #BOCO - Bob Costas [BGS*8]
Courtesy of COMC.com