"Matt
You were on full wets to their inters yes?
My guess is as it only just started to rain, the track was just damp, not wet, and thus your wets were not suited, compared to their inters. That would account for the 4s difference I think
I made a thread about this a while ago. It makes sense but it isn't right. I had a race at Belgium last season where my driver, for some reason, was 4.5 seconds off the pace in Qualifying.
It did start to rain after lockout yes, and when I saw the results I thought the same thing as you, "they must be on inters", but when the race started I was wrong, everyone was on FULL wets. So I was a little stumped.
Sadly the devs didn't respond to my thread so I was left unanswered as to what went wrong.
Also pays to note my driver was by far the best in my league at the time. He was also the only driver with a maximum speed graph and 19 wet where as almost everyone else had drivers with like 4/5/6 wet etc.
In the end I was 90% convinced that drivers aren't all on the track at the same time for qualifying, and that maybe some qualified in the wet and some in the dry? I really don't know.