First, it is unclear, what type of server do you have.
It is also unclear, that it measures the power on the cpu(s) or on the whole mainboard.
But the answer for your question is nearly the same in all cases.
From the power supply is coming a constant voltage, but variable current. There is a chip somewhere around the power connectors on the mainboard (or around the CPU, if it is a CPU voltage measurement).
On most printed circuits there is a chip on the power source which reshapes and "cleans" the input current, this measurement functionality is integrated probably into that. Next to the measurement, it can communicate the actual current data on an i2c bus with the other sensors of the computer.
The power usage is calculated probably by your bios based on Ohm's law.
The exact details would need detailed informations, but this is which seems relative clear from your question.
144 Watt isn't too few, if your computer doesn't do anything, then your CPU cores stay in HLT in most time, your hard disk heads doesn't move, etc. Start to recompile something on all of your cpu cores while your are doing a filesystem reindexing, you will see on the spot what happen :-)