I bet the uth has a small gap between it and the tub bottom and is causing some heat to be lost. Its not a big deal.
You can bump up the temp of the tstat to get the tub floor where you want it. Just do it very slowly and let it settle for a few hours before making another adjustment.
In a glass tank I have my tstat has to be set just over a hundred to give me a 88-90 hotspot because of the lost heat going through the cage bottom.
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