There are different versions of the games that casinos have for starters.. the minimum bet size is different between some crypto casinos. Lil Devil bonus buy is like wild flower (i think thats the game) where it will randomly trigger either normal or super bonus.
I think either its to do with licensing or maybe just offering what they think is best for their customers but as long as the RTP remains the same then I just think the bonus buy is worth the tradeoff