We have no evidence that humans can even know, much less that they generally don't. And we do have evidence that there are situations where the brain will readily construct an explanation after the fact which can't possibly be true (experiments with split brain patients where researchers tricked one brain half into thinking the other half, and so the brain as a whole, had made a decision while the action was taken by the researchers, and made it explain how it has made the decision)

There is no basis for claiming to know that people don't usually make up explanations like this other than when e.g. breaking the process apart and writing it down step by step during. But even then individual decisions are "suspect".