Why are these extensions, .html and .htm, different? If they have same function then why are they called separately?

Answer 1

"HTM is used only as an alternate extension to .HTML. This happens for some reasons, for example, in some operating systems, like the Disk Operating System and Window 3.X, they do not allow the use of four-letter extensions. Therefore, instead of .html they use .htm."

http://www.differencebetween.net/technology/internet/difference-between-htm-and-html/