BBstat is calculated based on team ratings and team ratings can be influenced by a few things. For example in a blow out your bench will play more minutes and you won't expect your team ratings will be as good. Also team ratings doesn't quite calculate secondaries as well. A big man with great outside skills isn't going to affect their outside team ratings as much but could be a reason why they were owning their opposing big man with bad outside skills. That's why team ratings can be quite tricky and misleading.
For private league games, some teams might have TIE their previous cup game while others played Normal. Enthusiasm have massive influence on team ratings and changes base on each update so that's why it's easier to have higher BBstat for B3 and private league games since they are in between league and cup games. But of course I wouldn't discount teams pulling such ratings in league and cup games.
An example of inside versus outside play can show why it isn't ideal as a tool at times. You can pull really good outside team ratings which can give great numbers in BBstat but can be beaten by the lower BBstat inside team rating. Look at a past national game
(18624). Without calculating I can tell we have a superior BBstat rating. Overall it's a useful comparison tool but I wouldn't rely on it too heavily.