The sensor of the Thermapen is in the tip. Perhaps the sensor in the probe is more to the middle? If so, you may think you are measuring the temps at the same depth, but may not be doing so? That would be consistent with the middle being hotter than deeper in the bird.
Perhaps the way you inserted the probe into the butt and then the Thermapen worked out to be the same depth for each sensor?
Jim - Another transplanted Texan
Former KCBS CBJ
Large and Medium Big Green Eggs , Black 18.5" WSM, Blue Weber Performer - Stainless, Green Weber OTG Kettle , Brinkmann SnP Pro, and a Stainless UDS. One retired Portable Kitchen grill.
Red Thermapen, Maverick ET-73, EdgePro Apex Sharpener.
Avatar is the original 1951 Weber Kettle