IB programs or schools in the US tend to be optional, often with selective admission (and with self-selection by students in any case). Since it is explicitly a college-prep program that is typically seen as harder than typical US high school courses, it is likely that students who are selected for or who self-select into IB programs or schools in the US are stronger than the general pool of college-aspiring students, so it should be no surprise that their college admission rates are higher.