A couple of things may be going on.

--When you have the probe in the tub, you don't know how hot the flexwatt itself is actually getting. So it could be much hotter than 100* in order to get the tub temps to 92. However, if this is the case, I would suspect either you have a somewhat thick layer of substrate that it is having to heat through...OR it is open beneath the flexwatt (at least partially) and you're having heat escape below the tub, making it heat ineffeciently.

--The other thing could be that you're putting the probe in an area where the heat is getting trapped around the probe and the t-stat thinks the whole pad of flexwatt is warmed to 100* when really it's just that little pocket of trapped air around the probe.