I am just looking at the pictures and trying to figure out why the back is warmer than the front. Sometimes I overthink things.
My first thought was:
This may make no difference at all but, ... For drainage purposes, most patios are sloped away from the house. Meaning the side of the grill that is closest to the house will be slightly higher than the other. Looking at the pictures, it appears the back of the grill is facing the house and the front is facing out, away from the house. I am wondering if the fact the heat deflector and area with the louvers around the water pan both (may?) have the back side slightly higher than the front, if that could impact the temperature gradiant front to back? I am wondering if you spun the grill 180 degrees in the same location you have it, would the front of the grill become the warmer side? On second thought I doubt this would have a significant enough of an impact on the temperature gradient front to back.
On second thought, I really think it is because the pellet bin is at the back of the unit. The pellet bin is acting like double wall construction and the pellets in the bin are acting like additional insulation between the walls. The front only has a glass door. The back is more insulated so it retains more heat and that it is why it is hotter. That is my official SWAG.