This is common and it says a lot about the culture.
We are grateful and humble when we grow and get promotions.
The reality is that in Jiu Jitsu it is rare that a professor is under-qualified simply because it is easy to test their rank out by rolling with them.
The point is in BJJ if you get your belt from a legit instructor there is a great chance you earned it.
Yes, many variables may exist and each belt is a case-by-case basis.
Whatever the case may be, in order to achieve a new rank you did something good.
It doesn’t mean you have to be the best competitor. Even just being a good person (who understands Jiu Jitsu well) and a being good teammate who puts in the time can give you a well-deserved rank if you simply don’t have standout talent or ability or even the luxury of time.
So you likely deserve to be at the rank you got. If you question it it’s generally because you care about your Jiu Jitsu, which says a lot about you.
In closing, don’t worry too much whether you deserve your rank or not. Just stay consistent, do even better and you will fit into it soon enough.
Royce Gracie once said “A Belt only covers two inches of your a**, you have to cover the rest.”