The temperature monitor on a mainboard is made to be somewhat precise in the range of operating temperatures (like a bit above freezing to somewhere in the boiling point of water). The freezer might be too cold, and the oven might be too hot
   Remember that instead of some high-precision, high temperature range industrial device a simple diode is used. Temperature is gauged by the perceived resistence in a certain current (or at a certain voltage). This is perfect as long as the rest of the device is working - or as long as the operating temperatures are ok.
   If I remember correctly, military grade integrated circuits are rated to work at up to 125 Celsius (and as low as some -40 Celsius). While this would be ok for the freezer, is by no means ok for the oven