Wet tyres in rainy weather are not comparable to Soft tyres in dry weather. Plain and simple.
The only reason you feel like this is because as a race goes on, the faster lap times get, as drivers slowly fatigue less towards the end of a race and become more in rhythm with the course.
Yes.. You are correct.. It is because of the lower suface contact..
I've spoken with Jack in our live races about it.. I also asked why tyres heat up on straights and cool down on corners.. He knows it's an issue and will be sorted when we move to the new system...