There should be no manufacturing variance between the two Saddles, Trees are made in a Jig and should fit the same, the only difference should be the length of the bars between the two, so that is perplexing.
Have you tested the Bighorn on your RMSH, is it too narrow also?
Saddles based on an A-Fork Tree or a Wade Tree have about 1" more wither clearance than Trail based Trees, so that may be something to look at.
One problem may also be the Shoulder size between the two Horse, if the RMSH has larger shoulders the problem could be there is not enough Flair in the leading edge of the bar on the Circle Y Wide, I have heard of that problem many times with that brand.
Pics of the shoulder area would be a help, stand at the Haunches of your Horses and take a nice closeup of the wither/shoulder area and post them
May all your Trails be happy and safe ones