All of the markers are merged into tiles and cached, and it is more efficent for the browser to just load these than it would be to create a separate circle or image for each marker. At zoom level 15 and higher actual individual markers are drawn (each one as a separate png image).
There are still two layers, but both just contain image tiles: one contains the base map, and the other contains red circles on a transparent background.
There are still two layers, but both just contain image tiles: one contains the base map, and the other contains red circles on a transparent background.