In most cases the .html page that will be the default. Depending on how your host has set up their server, only 1 of the 2 pages will be used and the other will be completely ignored.
I think if you put a index.html file and index.htm file on the server, the index.html file will show up as the homepage.
Look at your stats and see if .htm has ever been accessed.
The 3 character extension is a carryover from the DOS days when a filename could only be 8 characters for the name and 3 characters for the extension. (for backwards compatibility with older computers/browsers)