Bat's moisture content goes up and down all the time. Not by much but leave your bat in the car during winter and it may add an oz in weight. Playing with it during a heatwave and it may lose a bit of moisture. .
here's one for you then Saunders old chum. Buy a bat from the sub continents... lower moisture content (In theory) and leave it in your car over the winter - gains moisture content (In Theory). If you are lucky you will have a Mahoosive bat which will now last longer (In Theory). Ha ha If only the science and reality was that simple. You'd see car boots full of willow driving to and from work.
It's right though, the fundamentals at least. One bat I have was just under 2.9 when it arrived with me. Been in the back room where we get a lot of condensation when the dryer is going (the Mrs is dead for putting it there by the way) and the bat now weighs over 2.10 and it hasn't ever been touched
Note - The bat has been in there over 12 months so the weight increase did not happen over night. It's a slow process.