Power Spectrum of the Current of Compact Fluorescent Lamp (CFL) Compared to the AC Input
The simple explanation is that the Compact Fluorescent Lamp (CFL) acts as Non Linear System and hence generates signals with different frequency than the input.  

Another thought would be that the sampling channel as a whole has different response to those frequencies and it damps 50 [Hz] / amplify 150 [Hz] (Or both).

You take into consideration the whole chain until you get the digital signal.