Strange behaviour showing the battery level

Hello, I tried to transmit the battery level of a puck using NRF.setServices.
My source code is the following:

NRF.setServices({
    0x180F : {
    0x2A19 : {
      readable : true,
      notify: true,
      value: [ Puck.getBatteryPercentage() ]
    }

This morning the battery level was 58%, a few hours later it was 64% (using NRF Connect App). How could this happen and how can I get the real battery level?

Kind regards