It may be to do with the difference in frequency on AC power coming from the wall socket.
For example US is 60hz UK is 50Hz. It only matters occasionally -- like on old belt drive hifi turntables which used an AC motor locked to the wall frequency and needed a new motor or different sized motor pulley if you moved from US to UK.
You need to check what the frequency your power is locally, probably because the device you are using needs to lock to the AC frequency for accurate timing or something. The local frequency is often printed on the plates showing voltage on things like washing machines, fans, vacuum cleaners.