I was wondering about this as well. For the races of this week my drivers delivered quite different times during practice as well. Almost as if there's two time tables (like for quite different temperature values) that are roughly one second apart and every lap randomly chooses one of them for its lap time.
For example today:
Rossini
SS 01:17.992
S 01:20.715
M 01:20.715
H 01:19.682
H 01:20.708
Lehto
SS 01:17.996
S 01:19.146
M 01:19.504
H 01:20.772
H 01:19.636




