Hi CK,
Good question. The answer is basically this: wood and cellophane (which is a wood derivative) are far from impermable barriers to either moisture vapor or oxygen. I'll spare the chem-engineering-speak about diffusivities, diffusion coefficients, MVTR, OP and the like. The bottom line is that given sufficient time, the cigars will reach an equilibrium with the external environment.
As folks who age for the long term want their sticks to do so gently and gracefully, this slow approach to equilibrium and resistance to environmental changes is considered a beneficial thing. This is true for the casual cigarist as well except that aging is not as important a goal as is protecting the wrapper from the humidity fluctuations that occur when the humi is opened and closed.
Hope that helps.
[snapback]272199[/snapback]