Just an idea: perhaps the torque (both torque and horsepower are improved in this 2019 engine) available at the lower-end of the RPM-range supports similar efficiency to the V6 SC because the V8 SC uses less fuel to get the significant mass of the SUV rolling.
I did a cursory search of the new 518hp 5.0 V8 SC, it did not yield any illuminating details such as: cylinder deactivation, continuous variable valve timing improvements, or addition of compression ignition. JLR may have just improved cooling, supercharger efficiency, timing, etc in order to squeeze-out more power and torque?
Coming from Jeep JKUs, many owners had dramatic MPG improvements or at least similar MPG (depending on mods) when swapping to GM Gen IV and Gen V V8 power trains when keeping all other modifications the same.
There is definitely a smarter way to articulate this, so apologies for my crude expression. The added low-end power probably uses fuel more efficiently than a higher-revving V6 or V6 SC (many JK and JKUs utilize aftermarket superchargers) when accelerating from a stop, slow speed, uphill etc.
The fuel economy numbers you posted are just EPA estimates, and do vary based on a myriad of factors. My idea is that JLR simply hit a "sweet spot" of torque, power, and weight with the 2019 518hp V8 SC since the aerodynamics of the V8 and V6 models are likely very similar.
