No don't worry about the temperature fluctuating that much, but the temp should be the GROUND temperature. If your using a digital thermometer and it says 87 to 90 degrees ambient air temp, the ground could be as high as 98 or 100.

I usually check my tank set up with a Zoo Med Rheostat with a IR Temp Gun and even on the lowest setting its too hot. I have to have about 1 inch of air space between the pad and cage so the temp isn't too hot.

Although the ambient air temp was about 85 to 90, the floor was 102 to 104, way too hot without any space. If i had just used the thermometer i would have never known.

I always tell people to invest in a good thermostat and a Infrared Temperature Gun, these are going to be your best friend when you get new arrivals, change cages, or go on vacation and don't have anyone to watch the snakes for you.