It's completely random unfortunately, me and my group member have tested several theories on how the event works and nothing makes sense in the end.

We can only assume it's just random, regardless what BP you are or what level you are the odds are you're going to run into teams moderately higher than your or significantly higher than you. And on a good night you may run into people relatively around your matching BPs.

I've also found that if you're a server that hasn't got the cross-server matsuri/sage then doing bonds during that time will end up getting you lower BP players.