heat tape fluctuations in home made rack

I made a rack of melamine. It hold 4 28qt sterilites.

I was able to route and snake the 4 inch heat tape all the way through. However on the bottom i snaked it up the side as ive heard tje bottom shelf gets tje coolest. I have a "control" tub with a thermometer in it and have testes it against my herpstat and it is within half degree F. I moves tjat tub to all slots while teating to assure probe is taped to the same spot.

My top shelf is 88.5
Second shelf is 90.4
Third shelf is 93.8
Bottom shelf is 90.1

My herpstat os set on 95.1 and the probe is taped the the heat tape at the beggining of the roll shelf 1.

Why os third shelf so much hotter?

https://dallas.craigslist.org/ndf/pet/5786036579.html my control tub is empty. Is there too much temperature variancee? Or am i over reading into this? I cant post pics yet but i have a friend who made a rack and i used his measurements.

I found a rack that looks similar to mine here