The time factor is the likely cause, and it's based on the average time it's taken others to solve the problem so it's appropriate that it be included in the measure of success.
Incidentally, I'm pretty sure the problems are rated in the pool alongside players, and their rating adjusts every time an attempt (successful or otherwise) is made just as the attemper's rating is based on the outcome.
I just started using the tactics trainer, and I find I'm enjoying it in general. One thing I don't understand, however, is that I often lose points even when I solve a problem correctly. I am assuming this is because I took "too much" time? I find that unless I answer within 60 seconds or less, I lose points, correct or not. Since it's not unusual for me to take 2-3 minutes on a problem, I find my rating continually dropping, although I have a 75% (9/12) success rate. This means that I am basically going to ignore my rating and just focus on my success rate. In my opinion, it seems ridiculous to lose points on a problem for taking 2-3 minutes on it, when 2-3 minutes is NOT an unusual amount of time to spend on a position in OTB time controls, and esp. if you're just thrown into the position (not in the middle of a game). I always take 20-30 seconds just to survey the pieces, pawn structure, material count, etc. to get a positional feel before even looking at tactics. I also follow up my decision with 15-20 seconds of "sleeping on it", just making sure I didn't miss something, looking for in-between moves, checks, etc. This is how I play OTB. Unless the focus is on speed rather than accuracy? I guess this might be expected, given no one online seems to have patience for a 15-20 "standard" game, (unless it's correspondence). Just my 2 cents.