Why can I beat an above 2000 standard rated player in 960 Blitz chess but NEVER in Standard chess???

There's a simple difference. 960 chess is less popular, so less player are playing in this mode and the elo ranking is kinda inflated. If you play against let's say 10 different person, your level will be based in comparaison of those 10 people, if you play against 1000 different people, the type of players is much more diverse and you will face many different way of playing which your not always accoustomed, it can come from this.

Also in standard chess, the opening knowledge is much more important than in 960 where the most important is the comprehension of the game which came later for same elo levels. A 2000 in standard as a better understanding of he game than a 2000 in 960 chess. ( in my opinion )