Your quite right as each product although manufactured to the same layout and standards can and do perform differently to others especially when it cuts out with water only.You've got a really good point about crud buildup providing insulation and therefore increased element temperature.
Even so, if I remember correctly mine did it even when I first got it and was boilng water only, to calibrate the numbers on the dial... maybe it's just piece-to-piece variations on the thermostat and where the sensor is positioned internally?
In fact I hardly ever use the thermostat itself, as I'm driving it with an external controller and temp probe :-)
I calibrated my own to the midway points on the control stat and found mine reasonably accurate to a degree but I still used an external thermometer which is calibrated twice yearly and works well.
They are a great boiler and a sound investment for AG and if you move on to an All in One system they still have a use for sparging and a trusty backup.