Remove the voltage between RTN and NEUT in a power supply unit
The line filter has differential caps to ground , which has a known impedance vs f and causes the ground current you measured.  These are intended to reduce conducted noise below allowed limits. 

The equivalent impedance of Zc(f) of both 330pF should be the same as 6 MΩ above. This depends on tolerance of C, voltage and current spectrum (fundamental with harmonics).  Zc=1/(2πfC) ( you do the math)

IEC/ UL specs limit all designs to 0.5 mA for safety.

got it?

Consider why you can get > 50Vac by squeezing a 10M scope probe tip from stray line capacitance to your body (dielectric antenna).  Is this safe? and why?