My general feeling on them is they are pointless for cricket bats, and fairly haphazard in general. We have one for measuring concrete floors before laying sports surfacing. I once tried to get a reading out of a puddle on the concrete, it came back as zero.
To get any kind of reliable moisture reading, the measuring implement (usually a metal prong connected to the digital readout) needs to be driven into the material being read. As I assume you didn't bury the prong into a cricket bat, the reading will be nonsense.
Just to note there are meters that give a reading without the requirement to drive a metal prong into the material being measured, but these meters can only give a relative reading compared to another material of a known density and moisture content.