When we do worse than we expected, we usually seem to criticise the situation. That's something completely natural - we want to improve our confidence.
I think it may be a form of cognitive dissonance - deep inside ourselves we know what we've done wrong (or at least have a hunch), but we refuse to accept it or consider it. Instead, we blame the situation, the people around us, the weather,...anything but ourselves.
But what if we did? What if we learned to, at least for the start, ignore the situation and look at what we've done wrong?
We'd learn something about ourselves in a specific situation, we consciously strive to get better and we can decide what our actions will be next time.
If we realise that we can indeed do better (or different) next time, we're going to improve. If we instead conclude that there wasn't much we could do differently and had no effect on the outcome, we shouldn't worry. Just move on and try to do better next time.
Exams are an example, at least in my experience. We study in our way, we think we're going to do just fine. Come exam time, we're faced with the pressure of the situation and the pressure of ourselves - so much time went into studying, am I going to fail? What's wrong with me?
Then we go on to criticise the curriculum, the questions on the exam and the professors. When in reality, we can't change those things. We can't affect the situation, we can only affect what we do to do the best we can.
Making ourselves accountable faces us with reality, with a chance to do better next time. Making the situation accountable, doesn't. In other words, it's more useful to focus on ourselves, on what we can change, than on what we cannot.