I've had times where on TGTT, the race starts for me, but all the other cars were not with me. Then I exited the race, and went to "watch" and could see the cars were all going as if nothing odd had happened at all.
I've also seen races (various tracks) where people are talking about there only being 4 people in the race, and a completely different race order... and I'm seeing 8 people or more in the race. But the people who only see 4 people on their list, don't even see the other cars on the track. They might even claim they didn't cause a collision... because from their point of view, the collision never took place - but the rest of us saw it or felt it.
I don't know what causes this, but it MUST be a connectivity issue of some type. Either a PSN error, or some connection issue of individual players... hard to say.