Paul it is because measuring skin temp of the flue has little to do with what is in the center of the pipe. I am willing to bet the gas temp went up at least 100 degrees but the skin temp does no change that much due the boundary layer and turbulence at the surface of the pipe.
Glad you survived the cooking LOL. My now ex 3'd wife and yes 3 strikes and out there will be no #4, is the only Italian woman I have ever met that could not cook to the point scrambled eggs and bacon were confusing for her to figure out. Her mom Victoria was a kitchen savant 'the house of no bad meals' every holiday an event with a half dozen things to chose from as a main course and a dozen sides table set for 20 family members on average. When we remodeled the kitchen I told her I will lay it out with the exception of where the phone is and a small tv to catch the news while making coffee as I did all the cooking so as to not poison myself or her. She got to pick the phone as the only reliable thing she could make was reservations.