AO and AE are only virtually 640x480. They're actually 640x240 being scaled up.
Compare your screenshot to these ones:
Compare the sleeping Slig and the Mudokon sprites. The quality of the background art. It all looks much better, and not only in terms of resolution. (i.e. even if you take both images and scale them down to 320x240 you'll still notice the quality difference)
Playing Oddworld games at this quality of graphics would be awesome, and assuming these screenshots were at some point real in-game screenshots it's possible they include them in the Oddbox version.