I think it's because the AI use traction control. In the dry, TC isn't too much of an advantage. In the wet, the AI bots can be just as close to the limit as they do in the dry, while any human not using TC has to be very careful. Also, it might be that there setups are designed to work well in the wet, slowing them slightly in the dry.
As for setup tips, this is what I would do.
- With the exception of Monza, I'd recommend running 11 rear wing, 9-11 front wing. 9 is more stable, but 11 is faster is you can handle it (personally I always use 11, but it can also vary by car/driving style).
- Slide the springs 2-3 to the left, raise ride height to 2 front 2 rear.
- Slide rear toe 1-4 to the left, slide front toe 0-3 to the left. The more you slide it the more stable it'll be, but that doesn't mean it'll be faster.
- Camber, don't change. If your using French Raceworks setups the cambers are probably already good.
- Gear-ratios. Adjust 7th for top-speed, adjust lower gears based on traction. If you're struggling to power out of a 1st gear corner without spinning, raise the gear ratio, keep raising it until you're able to go near flat in standard fuel mix. Do the same for all other gears where you're struggling for traction. Make the rest of them angle so that it should look something like a slash mark / / / / /
- Brakes, you'll probably want the brakes to have less pressure, with more pressure on the front then on the back. Don't adjust the bias too much, probably just 2-5%.
- Roll-bars, I'm not sure. I just leave them the same, because I don't know how to adjust them properly.
I think that's all the settings. The gear-ratios are probably the most important of all the tuning. It's one of those things where the wrong settings can either cost you a lot of laptime, or cause you to spin too easily. It's not an advantage to have good gear-ratios, it's more a disadvantage to have bad gear ratios, and learning how to get good ratios for all conditions can be tricky. Then you've got to learn how to make great gear ratios, and that's annoyingly difficult in this game.
Anyway, hope this helps and makes sense