Wood has dielectric properties and pinless moisture meters use this to provide readings. The meter, placed against the wood, generates a radio frequency field extending a specific distance. In one version of this type of meter the moisture content of the wood registers via the power loss of the signal. In the other type the change in electrical capacitance according to the wood’s moisture content is measured.
The length of projection of the frequency field generated by the meter varies from model to model, but in an ideal world the field should project to about the centre of the board. The manufacturer fixes the field depth. This doesn't always suit the thickness of the board in the test, and a less accurate reading is the result. So, for example, if the field projection set by the manufacturer is ~13 mm, this is ideal for boards that are about 25 mm thick, but it's not optimal for say, 75 mm thick material. I think some of the more sophisticated meters may have a facility for the user to specify, i.e., adjust, the field depth to suit a particular need ... but I'm not 100% sure of that.
As to not knowing the wood species you're testing, that's a tricky one because, as you guess, these meters generally come with written instructions and guidance for making adjustments for wood species, as well as sometimes for things like air temperature. I guess all you can do is either improve your wood species identification skills, or perhaps you'll have to rely on WAG, ha, ha. Slainte.