Well now you've changed your argument from rank being more potential to faster progression being more potential.
Faster progression can be an indicator of having more potential (but the definition doesn't say that, fyi, but I agree anyway). It's not always the case though. Sometimes having a better environment to train can make you better than those who have better potential. For example a student who gets special tutoring ends up getting better grades than some lazy student closet genius. The special training helped the first case.
Nappa and Raditz had a better environment (gravity, better fighters, harsher climate) so they progressed faster than Goku who had the ease and comfort of Earth. When Goku had a small dose of that kind of that on Kai's planet he eclipsed Raditz and Nappa's entire lifetime of training.