Temp is cooler near the surface than in the middle. What gives?

[albow]

My first time smoking anything in my new to me pellet grill/smoker. I've read about the stall but something weird is going on. I have a 10.66# butt that I started by preheating the grill. It's now been on for 12 hours at 225 and is only just above 160. I can wait and stay up late so that's no biggie. btw, temp has been showing like the grill has been running average of probably 10-15 degrees hotter overall. But, as I search around trying to ensure I'm taking the temperature in the center, I find the temperature decreases the closer I get to the top, rather than being hotter. I know my heat source is at the bottom, but I thought it was supposed to cook evenly , or almost so, all around, with the temperature being cooler as you move the thermometer closer to the center. I actually have 2 digital thermometers i am using and they read the same on ambient temperature with what seems to be a correct reading. What's going on?

 

[Chasdev]

As the internal moisture transfers to the ambient air it evaporates and that state change from liquid to gas lowers the temp where the moisture exits.
Also sometimes the probes are too close to the bottom of the meat where you will pick up grate temps.
I still struggle with probe placement, I've been reverting to using these readings to let me know when It's time to start spot checking with a hand held probe.
Been fooled more than once and pulled early only to find the meat needed more time, which is when the kitchen oven gets a nod.

 

[BigW.]

If room is available I try not to cook directly over the fire pot. On bigger chunks of meat I will also flip/rotate if I can. I also generally cook around 275 so things get done faster.