I'm good at drawing anything I see with full accuracy, given enough time - but drawing stuff out of your head, with no reference, is a different skill entirely than pure representational drawing. I'm not very good at it yet, so because it's a weakness of mine I've been putting a lot of work into figuring out perspective, lighting, volume, mass and form of 3d objects on the 2d plane, entirely made up out of my head. In other words, no references - I can feel that it's using a different, more active part of my brain than the usual right-hemisphere dominance of representational drawing. Drawing like this is a much more active use of the brain, combining the rational critical deliberation of the left along with the more creative right hemisphere in a much more cooperative way than simply translating the optical nerves signals to the arm with as little intererence as possible. The signals for something like this, rather than originating at the eye, originate from the interior brain. The optical perception of drawing purely from imagination seems to be secondary in relation to the drawn image, rather than as in representational drawing, where every motion of the mark-making tool is governed primarly and purely by the reflex of the perception of the optic nerve as translated through the angle of the eye and the contraction of the cornea.
Or a different analogy - drawing from imagination is like writing from imagination, whereas 'drawing-what-you-see' much more akin to summarizing a pre-existing written work. Not transcribing - that would be a photograph.
Regardless, this is too long already, but I hope my point is clear; for me it is revelatory, and something I will be paying greatly active attention to in the future.