The lead-in. Maybe somewhere the reasoning is made explicit, but I haven’t seen it. My guess is as follows: route finishes are invariably (?) at arches (which also tend to be segment ends—i.e. sprint finish lines, kom finish lines, etc). You’ve maybe noticed that when you are first spawned into a ride (at the start of the lead-in), there are many other Zwifters sitting there as well, who’ve just been spawned into a ride. It’s better to have that mob of newly spawned riders conglomerated at the start of a (typically) short lead-in than immediately surrounding the official route start / sprint-kom-etc finish line.
Yes, the course complete notice. Yes, totally natural to expect that it would correspond to the advertised distance—in reality it’s the lead-in plus the advertised distance, since you don’t “finish” a route until you go under the appropriate finishing arch (which never moves).
One of the most frequently requested UI updates is that we would see a “distance remaining to end of route” indication while actively riding routes. I’d love it, and they already do it for races/group rides, so it shouldn’t be difficult.