When this happens, does your driver mistake value correlate with the fluctuation in lap times? Especially at lower levels they are going to be prone to a high margin of error.
In my case, it doesn't always. Often, the setup will be better in terms of Feels good/change by 1 or 2 versus by a little, but the lap time will be slower and the mistake value won't account for all the time loss. My driver tends to lose between 0.4 and 0.7 each run.
He also always runs his best lap in qualifying. Whatever his quickest practice lap is, he'll beat it in qualifying every time, sometimes by as much as a whole second. One recent race, his best practice lap was 1:31.4 (actual time), but then he ran a 1:30.6 in qualifying.