If the material contains more moisture than the air, water vapour will be lost to the air in the container.
If the material contains less moisture than the air, water vapour will be gained from the air in the container.
So the bread is relatively moist gives up water vapour to the air, the biscuits are relatively dry so gain water from the moisten air.
The bread does reach an equilibrium but its at the point that you consider it to be stale.
In the UK there was a court case argued around this point. A company Called Mcvities sold a thing called a Jaffa cake, they wanted to say it was a biscuit for tax reasons, the legal definition was that a cake would go stale while a biscuit would be softened.
