Because NBA season is 82 games, the net rating is better correlated with their performance than say NFL which is only 17(mostly 16 in prev season), so in NFL it's entirely possible for a team to win 60%/10 games and still have negative net rating but once you pass like halfpoint/42 games in NBA team that win 60%/26 games out of 42 will have positive net rating (not always though, the better benchmark is somewhere around 60 games, this is when team starts blatantly tanking as they can get clear picture of their position in the draft)