in my rack I usually have a couple degree difference. tstat set to 94/95 with hotspot inside tub at about 90/91 but the difference you are seeing is crazy. I cant think of any reason you would be losing that much heat between tape and inside of tub. I guess my recommendation would be trying a different thermometer to see if that changes anything and make sure your shooting the bare bottom of the tub idk if there might have been a small space between paper towel and tub