postUpdate only chnges the item in openHAB. It doesn’t send the command to the binding and further to the chromecast.
That’s why it reverts back. The binding reads the volume from the chromecast - which is unchanged - and re-updates the items.
You should use sendCommand. That one updates the item AND sends that value to the binding to be sent to the device.
Read further on the docs Rules | openHAB
