Well not willing to wait a day to try this out, I'll just post my best guess.
Changing everyday would be a 24 hour period either marked by the login time, or midnight. I would guess the login time, then it would make sense that every time you login it starts a new 24 period and thus puts up a new background.
You may want to try an experiment with one of the shorter time frames, say every 10 minutes, and see if it also changes the background whenever you login.. I am guessing it will.
Tom