Unfortunately, I think it's a burn
As a novice cham owner, I struggled with the same issue:
First of all, what kind of thermometer are you using? I was using a cheap BBQ meat probe at first. My basking temp read between 90 and 100, but my cham was still getting burned.
In a panick I borrowed a digital therm from my neighbor. This particular kind has a little probe on a long wire you can place outside and get indoor and outdoor temps. I placed the probe directly under the light, and my basking temps were as high as 115* (horrible!!)
Second: these digital therms will allow you to view the highest and lowest temperature over time in a given area. I live in a cold climate, so even with a furnace, there are huge temperature fluctuations in my house. I would think id have my basking temp dialed in, then the girlfriend would turn the furnace up 5 degrees and while the heat was blowing, my basking temp might spike 10 to 15 degrees. Because most chams are kept in screen, this leaves them vulnerable to swings in temps.
The point I'm making is that even though your bask is reading in the high 80's, depending on what time of day you check it; the house may be cool in the mornings and heat up significantly in the afternoon as the sun shines through. So your basking temp may not stay consistent.