I set up a regular tank-style enclosure for a quarantine spot and put a heat mat on the underside. I also noticed what youre seeing and for mine I think it came down to uneven adherence to the bottom glass. I placed a towel under the tank so that the mat was sandwiched between the towel and tank bottom (while also helping to hold the tstat probe), at which point I started seeing more even temps,though still not what Im measuring in my rack tubs.

I already didnt think highly of heat mats but now Im really looking for some alternate heat source.