“So, while maybe certain elite schools have admissions folks that are carefully weighing HS GPA and determining how accurate it is due to different grading systems, I don’t believe the majority of colleges are doing that.”
80% of the colleges in the U.S. accept 80% or more of the students who apply. So this question is ONLY relevant to the small number of colleges that have selective and national/international admissions pools. And those colleges absolutely know how to make judgments between different HS grading systems.
At those few schools, the adcoms literally do this thousands and thousands of times each admissions cycle. It really is pretty easy to do.
“but how can a limited admissions staff know how all high schools, even in a particular region of the country, are grading?”
Because it is their job to know how to do that at the HSs within their region. And the high schools provide copious data to the colleges to help them with that job. Every HS transcript today is accompanied by a detailed HS “profile” that enables this process.