V-DAC Power Supply question.

Sorry for the question but I know nothing about power supplies.

Why are people using the Pyramid PSU which is 13.8 V and 2.5 A when the original PSU should be 12V 500mA?

And why is the Pyramid advertised as 12volts 3amp?

I need to upgrade the power supply (I have none atm) but I'm a bit confused about the voltage and amperage.