Because he is doctor, he is not just a pirate??? Lol.

Anyway I get what you wanted to say, but his "doctor" abilities come from his devil fruit and help him in battles and also part of his power level.
That's my point, Luffy trained hard, Law took it easy, at least easier than Luffy, considering that both were equals prior to time-skip, Luffy know should be a lot stronger than Law.
Actually Luffy has yet to show his strength in this arc, Law did show more, so he could maybe made impression that he is stronger, but he shouldn't be. That's just impression.