Part of it is that Physics C is a second-year Physics class taken only by gifted/hardcore stem students. Physics1 has in many high schools replaced Honors Physics, and thus is taken without preparation and often by a population that just needs one physics class and it’s the only one offered.
This explains the difference in scores - NOT the difficulty in material.
Physics C should NOT be taken as a 1st physics course. Physics 1 doesn’t cover a lot of material - half of what Physics B used to cover. Honors Physics is actually a better preparation for the SAT Physics test since it covers more topics. But AP Physics 1 IS an AP class and shouldn’t be confused with an “easy” class.