If I was placing a bet on variances it would be with the tire and not the heat shield. Most 35" tires are not a true 35" depending on brand/model. Perhaps a combo of both.
FWIW I'm on a 35" Duratrac.
Unfortunately, JC and myself disprove that it is a variance in the, Tires, Rims or Pressures, with our 2 trucks, as we both have the exact same Nitto tires, same factory black rims, and even were running the exact same psi in the tires. We had been in contact prior to my truck arriving and he shared all the details of his truck and tire upgrade, which I followed right down to the psi in each tire, including the spare. And after I did the exact same upgrade on mine, and found the spare would not fit without it being angled, he shared photos of his fitting perfectly flat.
So unless his dealer modified his heat shield without his knowledge, the variance between our 2 trucks has to be with the heat shield from the factory. Which I would assume is with the mounting location of that shield, but I guess there could be 2 different heat shields they are installing. But without having one that fits and one that does not in my hands at the same time, to compare and measure, I am not sure how we would ever prove a variance in the shields themselves.