I don't necessarily have any ideas, other than the fact placing a probe too close to the meat or sandwiched in between multiple cuts (especially if the meat is cold) may be causing air flow issues.
I've been dealing with what I suspect to be similar issues as I learn on my UDS. On my first BB rib cook, I ran it with a diffuser and my Tel Tru and Maverick (which was calibrated) were within 3 degrees of each other at one point. I was thrilled.
Then, on a more recent BB rib cook, with the same set up, I think I had that same size differential as the TT was saying 250 at the perimeter yet the Maverick languished in the low 200's or so, with the diffuser but the Maverick was closely situated between (but not touching) two rib slabs. I think I'll probably take more notes going forward and simply learn where the Tel Tru needs to be for good butts, ribs, etc so I become less reliant on the Mavericks. I also think the air flow and heat dispersal, especially in a drum, tend to fluctuate a lot, especially without an ATC.
I also had what seemed like abnormal Maverick vs TT discrepancies when I cooked 2 butts in the top rack of my UDS, and had a drip pan on the bottom rack, plus the diffuser. I think the drip pan threw off my readings and acted as a second diffuser, but I needed the butts for a party and was afraid of a grease fire.