Actually there are significant differences between Z-Wave and ZigBee devices that should be noted. Z-Wave devices ALWAYS report the % remaining. So that mean the device itself is responsible for calibrating the remaining battery capacity and the battery voltage.
ZigBee devices on the other hand can report either the % remaining (in which case it behaves like Z-Wave devices above) OR it can report the battery voltage. In the latter case it’s upto the device handler to translate the voltage to a % and it should account for battery type (LiIon, NiMH, Alkaline, Lithium etc). Most DTH’s don’t do this well since LiIon batteries have a non linear curve. Plus things like load voltage can impact the curves which need to be handled by the DTHs’. It’s explained in more details on this post: [RELEASE] Low Battery Notification with Customizable Alert/Warning/Monitor, Configurable Thresholds and Device Monitoring Alerts - #72 by RBoy
It can also happen that the battery updates may be lost in the mesh which can then cause significant delays in reporting, so if you press the button and see the battery % change then it may tend to indicate mesh trouble also.