Comentario oficial
When we show the tiled maps there is a limit to how far they can be zoomed. Yes, you can zoom in to a JPG more but that is because it is allowing you to overzoom it and stretch the pixels out to larger than one pixel on the screen. The web maps are only allowed to stretch pixels to the size of one pixel on the screen.
Also, due to how the web maps are tiled, the level of detail available in the web map may be slightly less than that which is available in the native resolution files like the JPG and GeoTiff.