Meanwhile, a University of Chicago study found that HS GPA was more important than test scores for predicting college performance (generally for Chicago area HS students going to any four year college, not University of Chicago specifically)…
http://talk.qa.collegeconfidential.com/college-admissions/2174485-uchicago-consortium-study-finds-high-school-gpas-outweigh-acts-for-college-readiness.html
http://news.uchicago.edu/story/test-scores-dont-stack-gpas-predicting-college-success
https://journals.sagepub.com/doi/10.3102/0013189X20902110
Perhaps it could be that whichever of HS GPA or SAT/ACT scores that a college emphasizes less becomes the more predictive factor, because the factor that the college emphasizes more gets compressed toward the top while the factor that the college emphasizes less is more widely distributed across the range among the college’s students.
An extreme example: suppose a college uses HS GPA only and admits only 4.0 students, but with any SAT/ACT score. Then HS GPA predicts nothing (because they are all the same), but SAT/ACT score has some predictive value for college GPA. At the other extreme, suppose a college uses SAT/ACT score only and admits only 1600/36 students, but with any HS GPA. Then SAT/ACT score predicts nothing (because they are all the same), but HS GPA has some predictive value for college GPA.