* Consider a family where the highschool student has to work to help out with rent and food bills, so they miss school or don't have time to work on assignments. Compare this to a wealthier family where the student has all the free time in the world.
* Or consider a district that skews wealthier and tutors push the haves up into the 10%, and push the have nots below. Plenty of the have nots would have hit the 10% threshold if only they had money.
* Or consider being a minority in a very white district that isn't fond of minorities.
These are just a few I've read about off the top of my head. The idea of standardized testing is fair, but only if it is aware of inequities in the system. This has been a debate since the 70's. In fact, the popular TV show "Diff'rent Strokes" did an episode about systemic racism in standardized testing the early 1980's.
I'm not arguing if Cali has it right or wrong, but there are problems with using just testing.