Because this is an official re-release (read: improved from web version for publication). The art is touched up/made more crisp and defined, and where it seems either needed or just a good idea, some panels are redrawn, some are added new, and sometimes some can be deleted entirely. So far not a lot seems to be different, aside from the art touch ups and the addition of a few extra panels to make a scene more impactful. (Like when Tadokoro goes and starts to cry at Nikaido's compliment of her art, we got a bit of a redraw with some extra panels for that scene that the webcomic didn't have.)