[lumiukko]

But even the grades are highly contextual. I am unfamiliar with how much they vary across the US, but usually grades depend a lot on the university itself and the professors teaching or grading the courses. The prestige of the university may be taken into account as well. Perhaps it wasn't possible to go to a "better" university, due to financial or simply personal reasons.

Diversity is a really important aspect in building PhD cohorts and research teams - only choosing the best candidates based on grades introduces a bad bias that may even exacerbate the situation.