[Pete Parisetti]

I'm no expert but
1- Is your setup exactly the same? this difference in speed maybe downforce and gearing
2- and yes You do a perfect time is by shaving little time on every entry mid and exit of a corner, this on a entie lap translates to 6 seconds

This.
In that video the upper driver carries always more speed: at entry, mid corner and accelerate earlier. 
Also he has a much better top speed and gearing.
No wonder that in a track like Spa this translates in 6 secs difference.

Thank you Giuseppe. I obviously tried all manners to be faster on entry and to come earlier on the throttle, and the car flies off, either understeering on entry or oversteering on exit. Generally, I have part of the wheels on the outside curb on entry and I try to clip the apex mid corner - maybe not all the time, but most of the time - so I cannot be accused of not using the track. 

I saw this comparing telemery in Iracing: my line is within inches of the other guy, I feel absolutely at the limit, and the other guy is several miles per hour faster mid corner, carries more speed and obviously gains time. 

This is exactly the issue: how do faster people keep the car on track when I have the feeling of driving on soap? When I look at their videos and when I look at the fine details of telemetry, it seems to me they are driving a different car.