Tactics Trainer - Helpful or Unhelpful?

I've been playing OTB chess almost every day for the last few months. I've read a couple of chess books, and I'm fairly confident that my game has improved significantly (from a fairly low starting level). However, just as an experiment, I've been playing the Tactics Trainer each day, and my "rating" has persistently been going... down.

Now I've been thinking more and more about this, and getting increasingly annoyed by it. My suspicion is that either:

• I am uniquely incapable of learning and my contact with Tactics Trainer is making me stupider (i.e. de-educating me) or;
• There's something around the scoring system, tempo or presentation of Tactics Trainer that discourages score progress for non-paying members and is engineered to give the illusion that you'll do better if you sign up with a subscription.

Without wishing to flatter myself, I'm tending towards the second explanation.

I have noticed that the scoring system is opaque - if you get a problem right, you may score +1, +3, +5, occasionally +10... If you pass only just within the time limit, you may even score a negative score. If you get a problem wrong, you may score as low as -12 or -15. Even if you pass 75% of the tests, it's unlikely that your score will progress. I also have noticed that the first problem I encounter is usually the one I'm worst at - it takes a couple of gos to get into the swing of the problems. Presumably chess.com presents you initially with a harder problem, which you're more likely to fail at it, knocking down your score significantly. Then you can succeed at a couple more, and nearly get back to where you were before.

Come on, chess.com, play fair! In general, you get better at things that you do more, but Tactics Trainer seems engineered not to reflect this. It seems unlikely that I am actually getting worse at chess (though if I have early onset Alzheimer's, maybe you are the best diagnostic tool ever).

Oh dear. That's worried me now. Maybe I'd better go visit a doctor...