Powder builds up, makes contact with plates, causes resistance heating across the very top layer of the residue. Lowest specific gravity and greatest plate shedding go hand-in-hand. Betcha nickel if you autopsy that sucker that is what you will find. Don't forget, the bottom of the grid is a thermal radiator as well as the sediment. The bottom of the case with nearly an inch of sediment that is hotter than the hinges of hades on top can be near ambient temp. What you describe makes perfect sense and agrees with what I dealt with over the years.