Pianos tuning changes because the wood soundboard, which is being held in a dome shape by the wood case, actually gets bigger when it absorbs moisture and pushes up harder on the bridge, making everything sharp.Pianos really change timbre as well as tuning because the wool in the felts take up and give off water like the wood. I agree with you about a 5 degree swing either was is audible.
mugre
When humidity goes down, the soundboard actually shrinks a little, dome goes down, bridge lowers, and strings go flat.
