Because of the attack types.
BB wasnt screaming against Ace for nothing, why are you down playing Ace now huh? All to make Law look much better than what he is?
Again why is it a mid diff?
The fight literally just started, Teach hasnt used a single Yami technique on Law yet and you expect him to beat him already? He just used two quake moves, one mid air and the other to block a KROOM.
Just look at Rooftop fight with Dragon Kaido getting battered by Supernovas... I doubt you'd say any of it is mid diff bcs "Kaido didnt beat them already".
As i said,
Ace didn't show anything like this at all against BB.
The best thing Ace showed against BB was this, BB didn't have his crewmate come to him asking about retreat there after that.
Ace had been tagged a few times by that point. Law has not been once. Also, BB using Gura is about as same as him using the Yami as far as Damage and trying is considered. Gura is the fruit dubbed as the strongest paramecia.
And you yourself said that Ace vs BB was a mid diff. So, This fight currently is a mid diff too according to your own classification of Ace vs BB.
So, since BB has started using Yami now, Law will get tagged. But he will still land hits on BB as well. And if the Ace fight is anything to go by, Law will be landing more devastating hits as the fight goes on.