And why is this bias mostly helping Asians, a minority group, who more likely to be learning English as a second language?
And indeed, the definition of "best" in this context is circular.
I'm not saying you're wrong, just that you can't be certain you're right.
On most of these tests, you have to solve math or logic problems, or read passages and synthesize information. It's hard to imagine someone could do exceptionally well answering those questions quickly & accurately and not perform in a classroom, or be unable to do those tasks yet excel academically.
It's entirely possible to have the latter be true without the former.
As for your "It's hard to imagine..." in my career teaching I've met both phenotypes.
I agree that the effect is small if anything in today's society, however.