Trucks do drink even more than 50l/100km on some use and they drive quite a lot more than you in year, there is no point being worried for you, imo.
If you would use your daily ride on track similar way as you use cars in LFS, it would drink 20l/100km easily I would guess, depends bit of car, but it is loads more than what it drinks on street
My trackday car (former daily ride) is very good in economy, only ~32l/100km on track
Many of us had already done tens of thousands of miles before fuel was introduced to LFS so the ratio of fuel used for distance driven (based on LFS World stats) is not necessarily an accurate representation of true fuel economy.
My point is: fuel usage is not based on distance driven, it is based on throttle use (or revs, probably). I manually blip the throttle so I would most likely use more fuel per lap than someone who didn't blip. Wheel spin was mentioned too, and that makes a noticeable difference to fuel usage over a long race. Time spent revving in the gravel traps is another example.
Certainly engine has to be using fuel when idling, how else one could run out of fuel in first place? If LFS does not use fuel when idling then there is a bug.
If throttle is not touched there still is some throttle applied, via idle valve or some other way, fuel is also cut only if engine braking is strong enough, LFS should take these into account when calculating fuel consumption. For carbs it is different, but hopefully those can be forgotten from LFS