How is the output voltage of a mobile phone charger fixed to a rated value?
Those "wide range" chargers do not just have a transformer and a rectifier inside but some electronic curcuitry.
They use so called switching regulators.
So basically there is first a rectifier that rectifies the mains voltage so we have a 150V or 300V DC Voltage. Now we basically use a transistor to switch this voltage on and off trough a really small transformer (but at a high frequency).
The switching regulator measures the output voltage and controls the power transistor that switches the high voltage.

This way you can build a system that accepts a wide range of input voltages and still delivers a stable output. It's small, quite efficient and cheap.