Bongo Fury

The voltage will vary with the type of metal the probe is made from !
There are tables like this one https://en.wikipedia.org/wiki/Standard_ ... data_page)
which define the standard electrode potentials. Most meter probes will be some form of plated metal the exact plating however will vary and hence the voltage that appears.

All very true....also bare in mind that unless you have a quality voltmeter that has been regularly tested to standard and the individual error recorded, the chances of being accurate to measure +/- 0.4 volt are pretty hit and miss.