Power consumption
Why the 16W difference between Ryzen 3 1200 and Ryzen 3 1300X? The answer has to do with the voltage the chips operate at, as the former was automatically configured in the BIOS at 1.26V while the faster 1300X had its voltage bumped up to 1.38V. AMD is simply pushing more juice through Ryzen 3 veins in order to achieve higher boost states.
That difference, mind, is all-but wiped out when a discrete card is added to the equation.