hmm, it seems the art changes in many possibilities due to certain reasons :
1. The artist were not able to work on the drawing which perhaps work overload and it have been passed to the assistant/a newbie.
2.The artist is in a rush to submit the manuscript as the deadline approaches and one of the files were suddenly mixed(example chapter one draft were put inside the finished folder accidentally)
3.there is a certain changes made in one of the panels which should have included as we can compare, as example : the first artist were drawing the first chapter were introducing the MC and his parties , during recruiting the priestess , there should be a fight with the rude customers in that chapter but it was settled with third party interference only . Then out of sudden , the author decided to change the scenery by stating that a fight scenario should be drawn then followed by the lord/third party presence.Since the fixing request was an emergency , the artist were not able to do it since he was working with other things which resulting to rely to his assistant/newbie.