If you break it down to very basic chemistry and physics, it is always a function of the limited chemical energy that the carbons in your fuel can actually produce by burning them in order to move an optimally designed mass of an aeroplane through the atmosphere - wheras the latter will also be limited but dependant on a far greater number of factors, which can be made more efficient or compromised - the more mass to be moved or the more drag (e.g. due to bigger cabin size and other luxuries that the passenger demands), the more carbons need to be burnt in the most efficient way, by developing turbine materials and engine aerodynamics, that can handle higher temperatures - but even then you will eventually hit peak efficiancy and the gain will become insignificant unless we start developing fuels, that can produce more energy per molecule than current jetfuel.
So what I am trying to get to, is that even though the G600 is said to be optimized with more efficiant engines and aerodynamics- they must have compromised somewhere else, for a hopefully justfied reason, resulting in the unexpected information in the tables you have posted.