My theory is about this a couple of things
It depends on your rig and kit you use, and PC etc, if you are state of the art you can run everything max and see more detail, it's more immersive etc. Wheels, I would think most aliens and freak boys are using very good either custom or near wheel and pedal sets, this makes a difference as you can tune them to feel the rather crap basic FFB better. 3 screens, maybe VR, custom seats etc.
But mainly, it's down to what I call "clicky" setups. If you understand the game physics, and what the devs concentrate on, and most importantly the main things to adjust you will find a lot of time. You can faff about all day tuning aero, tyre pressures, etc, but if you dont know what the MAIN thing is to change, you wont find the time.
I have no idea what they are, but they will be able to find tenths per corner, be flat in places you can't be, and simply be seconds a lap faster. They are maybe a second to 2 faster than you, but not 5, that is ALL from setups, knowing what to change, and what the main things are they look for.
There are also smaller things like track cutting.
I am usually about 3 seconds off the freaks on the common fast lap runs they have, which I am happy with, I run an old PC with a G25, one screen and nothing else. And I have a life.