From Tim Bray (read the rest, too):
The time starts counting when you hit “next slide” for the first time; not when you hit “Play” on the slide show. Now, in many cases, when the audience comes into the room, you want to have your title slide showing so they know they’re in the right room.
Thus the recommendation. So you walk up to the podium, hit “Next Slide” and because there are two copies of the title, the screen doesn’t change, but the time clock starts counting.