This is just a guess, but I bet the system is using a buffered document of some sort to improve performance. This would be similar when you write graphics applications and use buffered images.

In graphics it takes a lot of resources to display and alter images in real time. So to get around the problem you build an image in the background, then display it. While the system is painting the screen with the current image it can start building the next image.

But this is just a guess.