The 840M uses about 15W of power not 30W. 30W is where the 750M/950M are already at excluding turbo mode.
Check the power consumption of the notebooks. They are more or less the same.
Look at the power consumption of the notebooks I posted.
28W CPU in 13" notebook ~50W
15W CPU+ 15W 840M in the same 13" notebook ~50W
Nvidia does not officially release TDP values but you can infer them from the notebooks in tests.
Well technically it is even lower than 15W if you go by nvidia. They state 17W for CPU+GPU in a game. Keep in mind in game the CPU will not necessarily need all its TDP. Also the notebookcheck tests show medium power consumption (3dmark11 run first scene) of 43W which is less than the full load 50W.