Sounds like you have already ruled out an issue with the two thermometers. I expect they are both accurate; a pork butt is a lot of cold protein sitting on the grate. I see the same thing from time to time depending on what I am cooking but they typically converge over time and sometimes are pretty close to equal.
I use a cyber q and still set the temp for the one I am shooting for. When it is heat soaked and before I put the meat on, they are nearly identical readings for an indirect cook.