I have been warned by entities that I deal with that if I ever actively pursue tarot as a public service I will have a reoccurring problem with my predictions being good until somebody comes to count upon them and then at the most opportune moment for maximum emotional devastation the rug will get pulled from underneath and the answer will be dramatically and destructively wrong.
Well, ravenest can ridicule your explanation as much as he/she likes, placebo, but the fact is, what you described here seems to be precisely what happens, to a lot of people I've spoken with....tarot will always get it right, until that time, when it has gained your trust, on that incredibly important occasion, when it suddenly lets you down and causes devastation....call it entities, call it the Devil's doing, like Catholics do, give a philosophical explanation or a scientific one with statistics and the laws of probabilities, but the fact remains, I've heard this story, exactly like you put it, dozens of times, and it really does seem like a perverted mechanism... very dangerous...I'm sure we will never get to the bottom of this question, no matter how long this thread goes on for, the most we can do is learn to take tarot predictions with caution-more than we ever did-as a pinch of salt...
And even ravenest cannot be sure that the explanation for this is logical, rather than crazy or " magical "...