In this picture I am holding a piece of metal up against the underside of the simulated deck/hull. As you can see the reading has gone from 6% up to 7.5%. This cored deck piece is a little over 3/4" thick and still the meter picked up the metal. If the deck were thinner the reading would have been higher.
It is very, very important to know what is UNDER the deck before taking a reading and the approximate thickness. Some new boats, from builders like Catalina, have aluminum plates glassed into the deck under winches, stanchions and other hardware. These aluminum insertions will cause "wet" looking readings as they are only about 1/4" deep at best. Large backing plates can also throw off readings so please do your best to look inside and to be sure your not reading metals through the deck.
This meter is designed to read approximately .5" deep but as you can see it will still, just barely, see a piece of metal at 3/4" depth. This is a good thing especially on cored hulls. Why? Well for one meters will read the copper in bottom paint as moisture so you can't use one over bottom paint.
Secondarily any hull that has recently been hauled will have rather high moisture readings. If the hull is cored, and close to 3/4 of an inch thick, the exterior paint won't throw off the meter by much and you'll be able to take moisture readings from inside. This is why it is best to go around thru-hulls from the inside not outside. On solid glass hulls I would not bother metering and would suggest focusing your efforts on the deck and bulkheads. If everything else checks out, namely deck and bulkheads, bring in your surveyor to do a full bottom examination.
One other area for potential high misreading could be a gelcoat with a high metals content. Some gelcoat manufacturers use titanium dioxide as a coloring/pigment agent and in some cases it can throw the meter off. If you get consistently high readings on a gelcoat find a known dry area and take a baseline reading to compare against.