Frosh typically have lower college GPAs than their high school GPAs, because the A/B students in high school get spread across the A/B/C range in college (and a few get D/F grades). At super-selective colleges, A students in high school get spread across the A/B range (with a few C/D/F grades). The adjustment from high school to college also affects some students.
However, college GPAs tend to rise as students move up class levels because (a) the students who have the most academic difficulty drop out or get dismissed, and (b) as students move up in class levels, they take mostly courses in their majors, which are their presumed main interest, compared to less desired general education and preliminary requirements.