Depending on how wide your fire basket is, a few inches will give you that much variation. A probe that extends under the meat will read much hotter than one sitting next to the meat on the grate. Try testing it it without meat on the grill with the probe sitting above the probe in the same location. If your using some type of plate or pan to disperse the heat it would also cause this problem, because the part of the probe closest to the barrel will receive more direct heat from below than your probe will sitting closer to the center. Just want to varify that they are reading the same in the cooker, tie the two together with a piece of wire the next time you fire it up. Direct cooking has hot spots, always has, always will, the UDS does a lot to eliminate these problems, but nothings perfect.
Weber Imperial Sequoia (restoration in progress)
Weber Platinum (The Green Lantern)
Weber OTS 22.5
UDS (with more to come)
Old Smokey ES (A gift, don't buy one)
Humble member of the Society for the Preservation of Authentic Royal Magical Rare Kaskaskian Peppers
Honor bound to protect, prepare, and serve