I think it's a matter of when they played skewing perception of their careers, and it goes both ways.
For example: Clyde Drexler form your original post, is a classic example of a guy who, if their career occurred even just five years earlier or later they're probably seen in a different light, because their career overlapped with an all time talent who overshadowed them.
Going the other way, Chris Paul gets regarded as one of the all time greats because his career has occurred in a span when there are few pure point guards. As a result, what he does has a unique spot in the sport in the perception of most people. Same for Harden. He just scored in a way that was different than what people were really used to in his prime. Whenever you stand out from everyone else in some way, it automatically boosts your career, even if realistically speaking, the achievements are lacking.