IIS2ICLX is an inclinometer targeting top-notch accuracy and repeatability performances, and ST wanted to provide a device where all the available axes perform at best and at the same level. Considering that physical structures of the MEMS which enable in-plane sensing (so X and Y) and out-of-plane sensing (so Z axis) are different, they behaves differently in term of noise, in term of offset and offset drift, in term of frequency response and so on. So, when you buy a 3 axis device, it is not really true that you can use indifferently any axis or combine any axis because your accuracy will be anyhow limited by the worst performing axis - which is typically the out of plane one.
