120V between chassis grounds of two different devices
The DC has a common mode AC from the SMPS capacitance across the insulation transformer.  The leakage should not exceed ~ 100uA shorted to earth gnd. This depends on Vac(f)/Zc(f)=Ic(f) and the leakage capacitance might only be 50 pF but at 50kHz or much higher. 

The "f" can be from the SMPS rise time, rep. frequency, PWM duty cycle and modulated by line f, 2f etc. so in general, we can only say it is not from the DC.;}

Electric sensations are not determined by the leakage voltage, rather induced current where  I=V/Z as a function of some frequencies.

These symptoms are common to all types of laptops and may be caused by the laptop earth gnd EMI filter noise reduction filter along with galvanic capacitance coupling of the SMPS RF transformer in either or both devices.

My cure when using the laptop charger is to connect the laptop case or 0Vdc signal to earth ground with a bigger RF Cap such as 10nF. 

A direct connect of Earth gnd to 0V dc also works but; may induce hum on external microphones , exposes you to ground-fault currents, since it is no longer "floating".. i.e. if you touch a live low impedance line to frame fault situation, it could be fatal.  A laptop VGA cable to an earth grounded Monitor also connects this 0Vdc to 0Vac earth ground on 3 prong plugged monitors.

This 100 to 200 uA leakage current can be painful to a knee or wrist but is not fatal.  But then the EMI filter is supposed to suppress reduces accumulation of functional issues on other electronic devices, somewhere nearby.  the RF transformer leakage is unavoidable unless by fluke  the leakage of 2 devices matched amplitude and phase to null the interface current of line frequency modulated RF current.

If you wanted to specify accurate specs, device models and accurate current waveform measurements, then a more precise answer is possible.

Final remarks

Unfortunately, this situation is not 100% unavoidable between two electronic devices each with (RF) SMPS AC-DC converters.  The galvanic isolation and grid isolation is guaranteed to some HIPOT test leakage current max and this protects you from grid voltages and stray lightning noise while EMI to grid in both directions and isolates you from live ground faults to some extent and all the while Low Pass Filters transients in both directions to the grid (both X (Diff) and Y mode -CM ).
  But it can create nuisance leakage noise current between earth gnd or other mobile charged units.
So you can avoid this situation, or shunt the current to a ground common to both devices with a small RF cap. (ceramic or plastic)