USC uses the SC UGP system. All of the high school classes here in SC will give you a number grade instead of an A-F. So your 6 classes can look something like 84, 88, 89, 92, 94, 95 (3 As, and 3Bs). An 84 will read as 3.4 for a core class, 3.9 for honors, and 4.4 for AP/IB/dual enrollment (although I don’t know if USC counts out of state honors in the same light).
Using that example, 3 As and 3 Bs (as long as each course is worth 1 unit each) would give you the usual GPA of 3.5. But with the SC UGP system, those grades would give you a GPA of 4.03. South Carolina has this system because of the cut-off in state scholarships.