Maybe the thermometer was some distance from the heating element and the heat transfer was by some slow process
- It takes a little bit of time for the heating element to reach a temperature which is capable of heating up the water.
- It takes a bit of time to raise the temperature of water since it has a large heat capacity and there is a significant quantity of water.
- The thermometer has a low response time which means it does not record changes in temperature very quickly.
- The thermometer has a poor sensitivity and therefore small changes in temperature cannot be easily detected.
- It takes a bit of time for energy from the heating element to transfer via molecular diffusion processes alone to the region around the thermometer.
Unlikely but possible:
- The thermometer may be faulty.
- The kettle may be faulty.