It gets worse. Unfortunately, some of these assumptions entail faulty logic. Not only that, but once you're headed down an illogical path, you're forced to periodically invent even more illogic to reconcile the contradictions you're already stuck in. If this is unclear, a simple example may help. Let's say I'm sketching a person touching his right shoulder with his right hand, and I'm drawing it without benefit of a model.
Errors trigger other errors.
This leaves only a small portion of your mind available for a critical look at the ongoing results. Later, when you're no longer doing the work, all you have to do is look at it. Looking is less complicated than doing, so more of your mind is available for critical judgment. Creating work you care about is an emotional act, and temporal distance results in an emotional distance. As I've already said, the process of creating something new requires you to generate and rely on a whole series of real-time working assumptions, many of which are unconscious and some of which are false.