Xpelair Fan Motor Controller Questions

[atlas1000]

Forgive my basic queries, but here goes. Here in the UK, Xpelair make quite a decent extraction/intake fan, for fitting in windows/walls. The controller offers full or half speed operation of the fan. This is done via a simple transformer in the controller. First, why would the makers use a transfomer and not something like a triac to drop the speed. Secondly, why does the temperature of this transformer vary. So, the fan may be running at half speed, meaning the transformer is switched into the circuit. The fan speed is therefore roughly half. So, why does the transformer temperature go up and down slightly during operation?