This is really fanfic-y but hear me out: 
At the end of the fight BB has Law beat and takes hi RP rubbings  
BB and crew board their ship and start heading off the island  
Law squeezes out one last Room, uses Scan to single out all the RP rubbings and just shambles them to himslef back on the island 
BB is ofc about to turn around but lo and behold the Marines are just off the horizon  
Thus BB has won the fight but loses the RPs 
Not to worry he has plans B through Z on lock 
Since he couldn't risk going back to finish off Law and take back the RPs because of the Marines BB picks his next target  
He has his own informants and connections, and also follows the news so he would know roughly where everyone is rn 
That is how we get either BB vs Shanks (who should have a few RPs lets be honest) or BB vs SHs  
 
The first match up would have BB killing Shanks as was theorized and the second would have him become take Luffy's RPs and arrive at Laugh Tale before him, as was theorized  
What do you think?