The following might explain why:
http://www.collegefactual.com/colleges/cuny-medgar-evers-college/academic-life/graduation-and-retention/#
Many colleges/universities require you to take a certain number of core classes as a prerequisite to graduating. They may include one or two courses each in math, sciences, literature, writing, history, etc.
Largely, yes, but it also depends on test scores, HS course rigor, and sometimes recommendation letters and other factors.