Bug?? AI Difficulty not consistent for same track (time versus lap based)

Wanted to try racing for only a certain amount of time versus specifying laps for each track in free play. I changed the setting to race for 15 minutes on Homestead and everyone was passing me. I changed the AI difficulty down to average and made sure everyone was in the same class and I still got smoked. After some frustration, changed the setting back to race 30 laps with laps being the finishing criteria not time. AI definitely raced like average racers after that.