When I was in college at UChicago, I dated another student from Appalachia. She once told me she had gotten straight As in high school calculus -- and when she took the AP exam, she got a 1, the worst possible grade. (And she was pretty talented, she later on pretty well at UChicago's math classes, which are not easy!)
It turned out, her school just didn't have the resources -- her high school calculus teacher had just not been qualified or capable of teaching the subject. And so, for years, her class was getting basically fictitious grades, losing time to god-knows-what in the classroom.
Of course, it's not possible to identify that this is occurring without standardized testing.
And when you remove standardized testing, you remove that basic barrier of accountability. It then takes all the way until college -- when the student is out of the educational pipeline -- that someone with a different set of incentives finally takes a look and then immediately discovers that the prior system has been essentially fraudulent, a potemkin village of academic performance.
The fact is that high schools are graduating kids with As and Bs in advanced math courses who haven't mastered foundational skills. The data from the UCSD report makes that clear. 20% took calculus in high school! Their GPA in math classes is ~3.6!
This is happening all over the country. Kids are over-reliant on calculators and generous grading policies.
Because the UC system does test-blind admissions, these kids look like skilled math students. (Applicants are not allowed to even submit ACT or SAT scores at UC schools.) Their foundational math gaps are only identified when they take the freshman math placement exam (no calculator allowed) and find out they aren't qualified for pre-calc or higher math at the collegiate level.