There are a number of issues with hydrofarms. I have looked very hard at them for adapting to an other use, extensive testing and many confabs with tech at hydrofarm.

The issues, well as mentioned, the foil tape is not helping. Flexwatt will also vary a few degrees in the length.

Hydrofarms have poor accuracy to start with they are spec'd at 2ºC up or down (that is just short of 4ºF) so set at 100 it could actually be 96-104. This is the beginning, they also have hysteresis this is the amount over and under the set point before it shuts down this is 1ºC up and down. This runs the possible error to 94-106º. The third issue is backlash swing. This is more complex and is compounded by cool room temps. It is the sample speed and the amount of time that big metal probe takes to respond. The heater heats up the probe heats up at a slower rate the unit shuts down the heater cools the probe cools and the unit kicks on again. The backlash is when the heater heats and cools faster than the probe. In the case of reptiles faster enclosure heats and cools. Cool room temps with plastic tubs are simply the worst for this condition. IMO they should NEVER be used on a tub with a UTH (carbon heater) connected to it with no gap. They are marginally ok with a indirect heat where there is a gap between heater and tub (like a rack) if used with an ACCURATE thermometer one better than 2ºF +/- (accurite units are 2ºC +/- for example they are not good enough the range is too great.) They are better suited to glass bottom glass acts as a buffer and levels the temp spikes.

In my tests I was able to get 15º spikes and 20º drops. With a tub direct heat, (wire type heating pad) in a room with an ambient air temp of 68ºF With a set point of 100º I got spikes of 121º with drops to 74º and back up again. Every 2 seconds. I personally would never recommend them I have had 8 units now in two groups (3 and 5) not one was even close to correct. I took the best 3 home as 'fail safes' these are they.
The best was 2ºF high and the worst was 5ºF high. If you don't have an accurate thermometer all you can do is guess.