Either the unit isn't actually earthed (so you're getting leakage as if it's acting like a Class II device instead of a Class I), or it IS earthed, and it's you who's charged up, and discharging to earth via the casing.
MANY years ago, a crude and simple earth test for a socket was to have a 60W bulb wired to a plug, from Live to Earth, and when plugged in the bulb should light brightly. This has long since ceased to be possible, due to 'modern' earth leakage trips.