Locality of ghost/phantom voltages?
The laptop charger is grounded and has an AC line filter to earth ground.  

The secondary is floating and the laptop is floating u less earthed by external connections such as VGA cable to an earthed LCD.

Since the earth line filter shunts common mode currents via a CMchoke thru the Y caps to earth gnd, there exists some stray capacitance across the ferrite transformer at the switching frequency which lowers the impedance of the small stray coupling capacitance.  

When your body is earthed by socks and leather shoes on wet grass or touching a stove, you become the return current path for this tiny leakage.  

Since the frequency is higher than line, you may feel more of a burning sensation than a 2f buzz if touched by a small contact area ( and thus high current density ) on the conductive corner of the laptop.  Knees are very sensitive to this I have found. It is just an irritant and not a safety risk, unless you have a quality control failure. 

When you disconnect the charger, the path is broken.  If you grasp the laptop while charging with a larger surface area, the sensation usually goes away due to lower current density.

Almost as bad as mosquitoes.