Best Fuel Mileage?

I drive a 2018 Pete 579. It has new factory standard tires. I use cruise control liberally. Never step fully on the gas except on steep mountainsides. Bobtail goes up to 12 mi, deadhead in a flat area takes about 10 mi, running 40,000 lbs load from Albuquerque to OKC is 8.3 mi with the AC running flat out. So I don’t get it why someone on YouTube said he gets out of his 2019 Mack truck (40,000 lbs load) only 5.3 mi. I drove a freightliner 2015 who did 8.5 deadhead and 7.5 with a 30,000 lbs load. Still not bad. So why are these other figures so low?