Junk on bottom of plate setter

After my last turbo cook of 2 pork shoulders (plate setter feet up) I noticed my foil missed a few areas and I had drippings on the plate setter. No big deal and I could understand it. What I found curious was a similar stain on the bottom of the plate setter which should have just been exposed to the fire. My wife thought it had a burnt plastic smell to it. I didn't think much of it but as I went to clean my plate setter I was forced to try and clean both sides at once which didn't go well. So my question is, how did the bottom of the plate setter get dirty? The only thing I can think of is that I put the plate setter on right after I started the fire starter and it was a waxy build up (but that should be easily cleaned by the pork shoulder cook) or it was chicken fat leftover from a previous cook (but that also should have been burnt off).