I'm currently having great fun with our immersion heater, which appears to be exactly the same model as the OPs.... indeed the thermostat is the same one.
I'm having repeated problems with the over temp cutout operating 'early'.
I'm still in the process of debugging this & currently have a thermocouple in the tank (yes, actually in the water) which I'm logging every 30s. The logging software will also record the max & min temperatures seen during the logging period.
I have the thermostat set at 60 deg & that temperature agrees with the logging data. It would appear to have a hysteresis of around 5 deg.
But, and here is the puzzle, the over temp cut-out still operates randomly. My next step is to log the current pulled by the immersion so that I can get a precise collation between the temperature of the water & the thermostat operating. My current suspicion is that the over temp cutout is operating way before its quoted design of 85 deg.
A note about that particular thermostat ... the over temp cut-out can't be reset until the thermostat is COLD. I thought I'd found the problem when I found the over temp cutout tripped & it wouldn't reset (tank is also heated by the central heating). I removed the thermostat ready to replace with another & decided to have one more go before I threw it away ... it reset.
The thermostat has been replaced, twice, so it isn't just a bad 'stat (although it could be a batch problem).