Welcome to the forums!
I'll start by saying I have zero experience with Zilla's heat mats so I can't speak to the effectiveness or reliability of them. I do think that that's a big difference. For example, I have both of my thermostats set to 95 with zoomed UTHs and the glass on the inside is 89-90. Are you using the recommended sized mat for your enclosure size? Also how big of a probe does your thermostat have? I noticed Zoomed thermostats are quite a bit bigger, and create a huge bulge in the mat under the tank, so there's less mat making contact with the glass, ergo less heat transfer. I initially fixed this with some duct tape, but later switched to a different brand thermostat with a smaller probe so more of the mat is actually touching the glass.
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