Wood is always subject to humidity changes, unfinished wood, like you find inside a soundbox will absord and loose moisture at a faster rate then finished wood.
If you take a freshly cut board, and let it dry in an attic, testing it every day for a few months with a moisture meter, you are being rather anul. You will also see that the moisture content of the board will drop quickly the first couple of weeks, then level out somewhat, but continue to increase and decrease a percent or two with changes in humidity.
Even if you test a very old hunk of wood, you will see that it absords and looses moisture with changes in relative humidity. In addition to building guitars, I also like making primitive archery tackle, where moisture content is critical to the success of a project, and I am anul.