My ex-girfriend's exceptional ability to draw realistic, well-proportioned humans in detailed scenes tipped me off to this phenomenon in much the same way.

I have very little ability to visualize a scene the way that must be required in order to do this. If I were attempting to draw (a pursuit I've long given up on, though I commend your attempt at overcoming the gap in your own abilities with music), I would have to draw an outline of the scene, and then come back and gradually fill in details, relying on my previous low-resolution version of the drawing for input as to how to draw the next iteration.

She was perfectly capable of starting on one end of the scene and filling it in at near full resolution. The proportion would be right in the end, requiring only minor touch-ups and modifications. She must have some very vivid image in her head.