Wood and wood products, even though they have been kiln- dried, are always gaining or losing moisture in an attempt to achieve moisture equilibrium with their environment.
In other words, wood will gain moisture in more humid locations and will lose moisture in drier locations. This gain and loss of moisture can become critical when it is recognized that wood swells when it gains moisture, and shrinks when it loses moisture.
In addition, wood machines and glues poorly if the moisture content is too high or too low.