Ceramic packages (like the ADXL203) are hermetic. The do not absorb or pass any
moisture at all. Plastic packages (all plastic packages from any manufacturer
as far as I know) are hydrophilic – they absorb water. This phenomenon is well
known, and is the reason plastic components are dry-packed and/or baked to
remove moisture before reflow soldering. After manufacturing the absorbed water
makes the package swell and changes the mechanical stress on the MEMS element.
As it dries out, the stress changes again…