Maybe if you're used to a certain pace... i.e. you're taking a long time with the easy tactics as you would more difficult ones and because the time to solve affects your rating it costs you.
Although that's interesting that between the two there would be such a large difference, I think easy tactics are much easier than hard tactics which are... well... hard 
I have noticed this before. It's as if the lower rated problems are on a different wavelength sometimes, and the higher rated problems actually seem easier. For example earlier on Chess Tempo I solved several 1900+ problems and then failed a 1600 problem (even though I didn't know each problem's rating until after I tried it).
Ya, it might just be that change of pase or awkward easy situation. Playing through a game would make it easier to catch a move because you know where the pieces are but if you dont it would be hard to see the (more) obvious sequences
I have been stuck in the 1400s a few times when I'm normally in the 17-1800's. I have noticed the same thing. Some of those problems just are harder for me to see. Haven't fully pinpointed what I'm missing in my tactical vision that many of these problems seem to have though.
After reading another topic recently, I reset my training scores and restarted, trying to increase the success percentage of the puzzles. I'm trying to train myself to see the solution before making the moves. I'm only up to 169 puzzles since the reset, have made it back into the 1800's, and have improved from about 50/50 to 60/40 on pass/fail ratios. Didn't get stuck in the 1400's this time around.
I've noticed that lately the problems have become longer, as they imply more moves, so I noticed I don't really improve my tactics rating, since it's harder to keep up with sometimes 4 or 5 moves ahead.
Maybe the system should keep 1-2 moves for those <1600, 3-4 for <1800 and >5 for those over 1800.
Ive quit playing chess on this account and made another, but since my tactics trainer is higher on this, i continued playing it(I dont play games on this acc anymore so dont bring up cheating). On my other account I started a tactics trainer too, but am anywhere from 200-300 points lower after a long while. (1900 max on this, 1630 max on my other)
My point: "Easier tactics" are harder than "hard tactics"
Does anybody else feel the same way?