In laymans terms (for I am a layman) the VSI indicated is an indication of barometric/pressure change which is "smoothed" by inputs from the IRU part of the ADIRU to make the I part in IVSI.
On a gusty day when you first align the ADIRUs have a look at the VSI jump around for a few minutes until the point that the ADIRUs settle down and take over the "smoothing"
All just based on observation and a limited tech knowledge.