I've noticed the same thing RedOak has, but I'd explain it like this.
Turn 10 worked on the A.I. modeling quite a bit since work started on Forza 1. In FM1 it was way too aggressive, so they toned it down, and I swear it's like they were reading my posts here. They created a number of bot drivers, drivatars, and gave them differing degrees of skill, aggression and other personality aspects. A few are even modelled after real world race drivers, to a point. I say that because while the bots in Forza 1 were like children, evidently they've only become teenagers in Forza 2 according to head guy Dan Greenawalt.
So it seems that not only do these drivatars have unique personalities and tactics, they also have "moods" to keep them from driving every race the same. Sometimes they leap off the line, sometimes they're a little tired or distracted, and lag back. Likewise, these moods seem to be affected by the way you drive around them. Sometimes they may not like you trying to edge past them and bump you around a bit, especially if they have the lead. They may take a disliking to you if you roar past them close. They all seem to surrender and cruise if you get a 4 or 5 second lead, like they did in Forza 1. Usually, they let you get by them if you have your nose past theirs, but this isn't always the case. Another thing to keep in mind is that these drivatars are set at moderate difficulty. It'll be interesting to see what they're like on hard.
Things may be different in the release game, but it should be much like the demo.