Q: odd DVI experience, connecting to projector

Two days ago we held a 90th birthday party for my father-in-law. I have a question mostly for curiosity down below, but it might be helpful to know the answer for those others who go through the same trauma.

In the morning I went to the venue to test the connection between their projector and my MacBook Pro. Hooked the projector up, using a DVI adapter I had brought...it worked perfectly. I did notice that the MacBook had automatically reset its own resolution (done a step or two), but thought "How nice; it takes care of all this on its own." So I went home, finished all of the iPhoto-based slideshows we were planning.

Went back in the evening, and hooked MacBook to projector, expecting a similar "instant marriage" between the two devices, and instead got ... what was a mystery for a while. And a short-term trauma, as we were desperately worried that we might not be able to show these slides over which we'd labored for so long.

Turns out that my MacBook Pro had the first time, during the morning test run, automatically gone to the "mirror mode" where it shows on the projector the exact things that were showing on its own screen. In the evening, however, it defaulted to the mode where it assumed this was a second display that I was hooking up ... so I could drag the iPhoto window off to the right and it would show on the big screen. Fortunately, I discovered that (through a call to a friend who's done a lot more with this kind of connection) and reset it to "mirror mode"

MY QUESTION: why would it have automatically gone to mirror mode one time, and then not done the same the next? I'd done nothing in the interim with "Display" settings. Any ideas? How does one stumble into such situations?

I did see somebody else in another Q&A refer to "Command-F1" as perhaps doing something relevant. Where in the manual(s) would I find these function and command key short-cuts listed?