All manufacturers do the same thing because it cost money to get the government certification. To do this for every conceivable configuration that could effect fuel consumption would be time consuming and expensive. I suspect your comment on tires was spot on. Take the Rebel's stock tires off and install 18 inch wheels and passenger grade tires and I'm betting the EPA figure would be met and probably exceeded.
The last I knew they run the EPA certification cycle for two wheel drive and four wheel drive for each engine combination.
Best regards,
Dusty
2019 Ram 1500 Billet Silver Laramie Quad Cab 2WD, 5.7 Hemi, 8HP75, 3.21 axle, 33 gallon fuel tank, factory dual exhaust, 18” wheels. Build date: 17 July 2018. Now at: 030139 miles.