Hi,
This is quite normal.
1) Your charger has only two connectors for power, thus it has no reference to "earth".
2) your charger is a switch mode device with caoacitive coupling between primary and secondary.
3) to attenuate coupled switching noise there are capacitors between primary and secondary of your charger.
One can not avoid capacitive coupling. And since there is high frequency switching on the primary side. To comply with EMI/EMC regulations one needs to suppress this switching noise coupled to the secondary side.
This is done by the primary-to-secondary-capacitor. It creates a path for the HF back to the primary side. While this reduces the secondary common mode HF ... it sadly creates a path for mains frequency.
So there is voltage (mains frequency) between secondary and earth. But the current is limited to a non dangerous value. Also - because of safety reasons - the capacitors need to comply with safety "X" standards.
In worst case there should be half of the mains voltage between secondary and earth.
But don't be scared of the voltage. As soon as you touch it the voltage will go down.
Mind: Not the voltage is dangerous, but the current.
The current should be below 0.5mA.
******
Better solutions:
* power supplies for medical use will have reduced stray current
* power supplies with earth connector are able to reduce the "voltage against earth"
Klaus