I’m btw 20 and 30 ftp difference (indoor reads lower) Same bike (literally wheel on trainer), same power meter, etc. I do think that heat plays a part, but maybe not that much as I keep my bike in the garage and during the winter when it’s 40f (and sometimes using a fan), I still have a descrepency.
I’m not an engineer, but I wonder if it has more to do with the fixed position of a trainer and the angles created while fixed, versus on the road where you have some slight side/side motion.