I have a Chess.com tactics rating (default blitz setting where time to solve is instrumental in getting points) hovering around 1750 but for some reason I can't get beyond 1550 on the Chess Tempo's Chess-Tactics site on the standard rating scale (no time limit). Any reason why? Is it only because the blitz capability and the no-time-limit capability are different, or because the problem difficulty ratings in these two websites are different?
Different sites will have different rating systems, even for games!
So, for tactical puzzles, basically every site works in a completely different manner. Also, I don't know about chess.com, but in chesstempo, different settings will also result in different ratings, as the kind of puzzles will be different, and the balance is not perfectly achieved.