This is common for many comic books adapted from TV shows and movies. They have photo realistic artwork for the covers, accurate to what was on screen. But with interior art that is much looser and stylized.
Part of this is marketing. They are trying to sell the books to the fans of the original source. Also it is VERY, VERY, hard to maintain that detail and accurate facial likeness over the many drawings needed to tell a story. But easy in a single image.
Still though for all the reasons I think this approach is a mistake. Because it just creates a false impression of the content of these books and alienates the audience they are trying to attract in the first place.
Part of this is marketing. They are trying to sell the books to the fans of the original source. Also it is VERY, VERY, hard to maintain that detail and accurate facial likeness over the many drawings needed to tell a story. But easy in a single image.
Still though for all the reasons I think this approach is a mistake. Because it just creates a false impression of the content of these books and alienates the audience they are trying to attract in the first place.