Yup, Glen has it right - moisture meters only give a 'corelation' to
moisture saturation. With such 'ionstrumentation' there is always the
question ofr calibration, resolution and accuracy/knowledge of the
operator ..... just like AN Y other instrument.

If you have any science/engineering or 'lab' background ...
There is another means to determine moisture saturation if you have a
sensitive 'balance' scale (the type used by chemists, etc. that can
read to .0001 grams): Use a desiccant (activated alumina or silica gel
and condition it by baking for 8 hours at 350 degrees (will be at -40
degrees dewpoint), let come to repeatable weight in the balance,
determine the mass, and put it under a film envelope securely taped to
the spot that you want to investigate, let sit for several days and
measure the moisture uptake with the balance. This is a direct method
and requires little 'interpretation'.

An easier 'correlation' method would be to used a known mass of silica
gel that has a special indicating dye, condition the silica gel as
above and simply measure the time that the gel turns from blue to red.