Will 2 multi-port USB chargers overload a 13A socket?
You're comparing apples to oranges. You're using the power output, to try and determine the input.  Without knowing about the transformer, and other circuitry, that's going to be quite difficult.  According to the specification listed by the manufacturer


  


The unit will draw about 1.4 amperes at 100-240 volts AC (VAC), which I'm assuming is the value when the unit is fully loaded.

The 12 ampere value you're using, is the output current which is at 5 volts DC (VDC).

This unit takes in AC power, steps it down and converts it to DC power.  The devices that are connected to it, then draw DC power to charge. The unit seems to be able to supply 12 amperes at 5 VDC, though it's not likely all the connected devices will take advantage of the full available current.

The 60 watts, is the amount of power available on the output side of the device.

12 amperes * 5 VDC = 60 watts