They are separate images and developers are permitted to vary them. I think many just forget to update their Marketplace icon. Live Tiles are larger than icons, so I'm not against making use of the extra space.
As it stands, a developer makes three icons. One for the app list, one for the Marketplace listing (not 100% sure on this, but earlier posters seem to have confirmed this), and then one for the Start Screen. Like on the Start Screen I might make the app say 1Radio across the top, but on the app listing, it'll say 1R for short, to use an example. Of course, when I did it I just had it say 1R on each.
I don't know, maybe in the future for WP8 programs there will be even more, because the tile icon for the start screen may split into two (small tile and large tile), with even more opportunities for confusion