Why is there a voltage between common and earth in light switch?
Common (neutral) should be the same voltage as earth (ground) if the wiring is done correctly. If it is not, then my first guess is that the wiring is wrong. Someone wired hot to neutral, probably.
A light switch will work if the neutral (white in U.S.) and black (hot) are swapped as in hot direct to light and neutral back to switch from bulb. The neutral will be hot when switch is off but will not if bulb is removed or light is on. This assumes neutral screws snug at breaker box. The hot wire is supposed to be switched.
Common is supposed to be connected to earth at the electrical entrance to the house or building.
Depending on the country / age of wiring, the wiring of light switches varies considerably. In the UK, the modern method uses a twin and Earth cable to send live down and back to the light switch from the ceiling rose. Thus one or both of the conductors can be carrying live (depending on the switch state) even though one is the colour for neutral. Either way, the most likely thing is that a live or neutral cable is touching the metal case of the switch and the earth has become disconnected somewhere in the the circuit. It is also possible the earth has come out somewhere and dropped onto a live connection. This sounds more likely because it is just one thing but due to the layout of electrical parts I don't think it is so likely.
That reading is correct. It's probable that the person getting shocks is building up static electricity and this is discharging to earth via the switch screws. A carpet with synthetic fibres and ladies with synthetic attire will be susceptible. The current dry weather will also promote this effect.