I’ve seen this.
I have a standard orange Rubbermaid 10gal mash tun with a SS false bottom. The thing is, if you take a temp, say with a digital thermometer, just after dough-in and a THOROUGH mix - from the center top of the grain - it is my opinion that somehow heat rises to that particular spot of the grain bed. Temp will rise in the center more than if I take an initial temp at the side. Also, I can cover the mash tun tightly and an hour later get a reading 1 degree higher than dough-in (??? - I thought I had the best mash tun in the world!!), there at that center point. BUT, when you mix the mash, your overall temp will decrease 4+°. The lower/upper outer/center temps are vastly different over time, in spite of cooler insulation.