Why are my tactics ratings on chesstempo and puzzle rush different?

After reading the comments, now I understand the question. The answer is simple: time. On ChessTempo you don't have a time limit, while on Puzzle Rush you have to answer within a few seconds if you want to make it to problem #25 or above. You also get a 1500 ChessTempo rating by getting right a certain amount of problems at that level (if you get them every single time your rating will go above that), but on Puzzle Rush you can only fail twice, so you'll need an effectiveness bigger than 90%... while answering within a few seconds!!!

There's also the difference in rating systems (each site could assign different ratings to problems of similar difficulty), so, as I wrote in the comments, if two numbers mean different things, you shouldn't expect them to match, despite looking related in appearence