All I know is that in the old versions of ZQ, there wasn't any static. And for each new page of tiles the filesize would get bigger even if the page was blank or mostly blank. I assume it's because each tile was the same colour-depth, even if it was left blank. So in the new versions (and this is just my guess) it only saves individual tiles that are in use, instead of saving the whole page.
This is to reduce the filesize of the game, obviously, not the filesize of the screenshots.