I observe the same difference in my rankings:
Chess.com's Tactics Trainer: around 2000
Chess Tactics Server.com: around 1350, success rate 89%
Chess Tempo.com: Standard around 1900 & Blitz around 2000
In CTS the main factor is the time: if you solve correctly but not quickly enough, you lose ranking points. Actually I do not concentrate on time there, but on the correct solution. I like very much CT Standard, you can sit down and think as much as you wish. For the same reason I am not a big fun of Tactics Trainer since the time you have implicitly shows the difficulty of the problem.
My advice is that you should solve problems from all sites concentrating mainly on correct solution. This will lead to big improvement in your results. Also reading books about tactics (Polgar, Weteschnik, Seirawan) can be very helpful.
I started using tactical servers this past year and have found them to be most helpful for improving my game. I don't have a lot of free time and I'm on the low end of the Ranking Spectrum - 1200ish. To get the most bang for the buck I spend a few hours a week just learning tactics. I'm trying to work my way up to the 1800's (tactics wise) before investing my limited time in learning openings, middle game strategy, etc.. Here are my tactical rankings:
Chess.com's Tactics Trainer: 1634
Chess Tactics Server.com: 1230
Chess Tempo.com: Standard 1657 & Blitz 1609
I'm not really sure what accounts for the disparity between sites. I originally started on ChessTecticsServer and have Rd 26.4,Tries 2574,and a Success percentage at 87.6. It seems quit hard to raise my ranking and I'm not sure if I should just create another account or if their ranking system is different from the others. I would like to know what others think about tactical training and what your collective thoughts are on them.