That's just another proof of Luffy getting the royal treatment, the other Supernovas never had a chance of being serious competitors for the One Piece since the race was already set up for Luffy.
My point is that just because Luffy accomplished more in a shorter amount of time, that is not what automatically puts him above guys like Kidd. He accomplished things faster because he prepared for 2 years before going into New World so he would have less struggles when he got there unlike the others who went straight in. But guess what Kidd was getting while Luffy was fighting monkeys on an island? Actual New World experience and probably having his haki bloom in intense battles.
To me it seems that Oda wants to show that Luffy got stronger the safe way while Kidd got stronger the risky way. Luffy reaps the benefits of his 2 year preparation by being able to have immediate impact upon entering New World and suffering no great losses, while Kidd struggled, gained scars, and lost an arm to get stronger. In the end they are still on a similar level through their different methods of training imo.