I was recently looking at the admission stats of some colleges and for this example, I will use UCI. Their average admitted students GPA is around 4.1 but their average SAT score is around 1750. I also looked into individual majors and even then, the SAT scores for enrolled students in every major was around 1750 and almost all of their GPA averages are above 4.
It made me a bit curious because 1750 is by most measures, not a good score but somehow a majority of them were able to achieve a GPA over 4.
GPA represents how you perform relative to your peers in your high school, and as Brajia stated above, high schools vary. The SAT roughly represents how you perform relative to all potential college students.
So UCI gets a set of students that are strong in their schools, but somewhat weak relative to the population of available students.
It doesn’t really mean anything for the average GPA to be 4.1, since they’re all on different scales. You couldn’t even get a 4.1 at my high school because they didn’t weight grades. 4.0 doesn’t mean “perfect” or even necessarily “good” if it’s possible to get something higher.