Managers: how do you grade / rank people under you

Received unexpected (but good) news that I was graded B for my performance grading. Not an A, but really way above my expectations.

I was expecting a default C for average performers as I have always been a bit quiet and non flashy. To be fair to myself, I believe I have always gone above and beyond the bare minimum.

However I can’t help but feel an imposter syndrome that I am undeserving and they made a mistake. I thought some others should’ve gotten the B instead. (From public service and there are quotas for different gradings).

Question: as a Manager, do you grade by capabilities or how shiny a person (maybe they took on a lot of additional responsibilities or side groups)?