I have the same issue with my 2 22.5" WSM's . My temp gauge on the WSM's are right on with the digitals until a few hours in and my mechanical gauges on both WSM go to 190. I believe the digital temp than the mechanical all day long. My buddy has the same issue with his 18" WSM. Can never always believe the mechanical gauges.
2- 22.5 WSM , 150 Gallon Offset , 250 Gallon Reverse Flow