It's a sloppy artistic detail, really. I mean, all that was needed was ooooone little line. Just one little horizontal line right under the mustache, and you WOULD have a cartoonishly high chin. Even I could have drawn that mouth properly. But no - the artist decided the page was good enough, and went to play video games instead. And as a result, Norman Osborn is descended from a line of mouthless horrors. I don't know whether to scold the artist or thank him.