There could be a number of things. You have made a micro climate in the tank in fact 2 micro climates, possibly more. It takes very little some times to alter the balance. I would suspect that near the window there is a heat source? Or it could be solar gain, or.. It is different, the issue is to solve the current problem not fret over why.

I would suggest a simple cheap option a canopy (diy) with a fluorescent light under the screen top. They kick a fair bit of heat with out ever becoming too hot. In fact they sit around 100ºF usually compared with RHP that often are in the 120 range. I have seen someone do this using simple zip ties. A deep blue aquarium led light can be used to help hold night temps. Led do get really hot but the electrical clusters around them can warm up more than you would think. Mine runs mid to high 90s.