No I've had same issue on PS4 lap time dropped by over 10 seconds. I thought I had wet tyres on but I still had soft tyres on.
It appears (although not to the same degree) to affect the PS4 as well.
I would assume, considering the AI is running a simplified physics system, that is the reason why when the system is under heavy load their times remain static, where as the player, who is running a much more complex physics system, and the combined load this is putting on an already taxed system, the player cannot attain the same laptimes as if the game wasn't under such a load from weather or AI numbers.
Ugh. :/
Odd that SMS didn't really address this yet in the forums, by the looks of it, as it seems to be a major, major bug. I hope this is patchable and will actually be patched in the near future.