If it goes back to normal after a while, without you changing it either intentionally or accidentally, implies that the server admin is running something to stop people having world-writable files. Odd, but I can see the use for either security, or people asking "Why can't I see my .html files in my browser?" when the file's not world-readavle.

Just as an experiment try and set the permissions to something more restrictive, such as 000. You won't be able to view the file anymore with a browser, but you can see if there is something odd with .html files.