I've been reading numerous posts (on other forums) about people having this same issue, and their general theory is that the results are scripted (the AI timing is fixed, the cars you see driving only pop-up to serve as obstacles but are in no way related to the timing. They also use this theory to explain that some cars aren't going into the pit-lane at all).
I haven't played the career mode that much yet, so I can't confirm this first hand, but it seems you're not the only one getting these strange results...