I have had that same experience with my WSM's also. My dome temp is about 20-40 degrees lower than what my guru reads. I trust the readings that my guru gives me and the guru is always right. I even use an oven thermometer to make sure. Almost always, the oven thermometer and the guru have the correct readings.
I never go by the wsm's dome temp!
[FONT=Arial]"Original" Oklahoma Joe Smoker [/FONT]
[FONT=Arial](2) 22" WSM [/FONT]
[FONT=Arial]Super fast [COLOR=lime]Lime Green[/COLOR] & [COLOR=pink]Pink[/COLOR] Thermapen[/FONT]