It's my understanding that, at least for elite private college admissions, the extremely high quality of college admissions counsellors at decent-or-better prep high schools is a major advantage. They know admissions officers by name at several schools, they know which rules are firm and must be followed and which can be bent or ignored to the applicant's advantage, they write excellent recommendation letters, they give highly tailored and accurate advice on getting into specific schools, the do a good job of focusing their and the students' efforts on schools they actually do have a good chance of getting in to (and also not aiming too low) and ruling out long-shots early to improve chances (because more, better-focused effort) at the remaining options, that kind of thing.

Probably less of a factor at (most) state schools, though.

It's much worse than that - there is whole industry around college admissions consulting. Why depend on your high-school counselor when you can hire an expert? In the worst cases the expert even helps students effectively pre-plan their whole high school experience to increase students' chances of getting in to elite schools.

In my country, universities don't compare school grades apples-to-apples. Usually if a school gives out inflated grades, that is taken into consideration.