Every model has a bounding sphere except stratmap ones. Basically, all what can be used in a battlemap, must have a bounding sphere: siege engines, mounts, soldiers, captains, generals... everybody. And yes, 99,9% of the non-Vanilla models around there have an incorrect one... and this is the result of the bad practice of using comments for new models taken from similar existing models. If you change only one extreme X/Y/Z axis extreme vertex, then you invalidate the bounding sphere since:
Given a non-empty set of points of finite extension in a 3-dimensional space, the bounding sphere for that set is the smallest 3-dimensional solid sphere containing all its points.
Spoiler Alert, click show to read:
Hence, if you take a soldier and you change his weapon from a sword to a pike, you have invalidated the current bounding sphere and he will start behaving in a silly way.
If you want more information about this, I just wrote it down in the Technical Details spoiler of the first post. My tools can acually recalculate (hence, fix) the bounding sphere of a model during the conversion from MS3D to MESH. So basically, if you convert a MESH to MS3D and then convert the resulting MS3D model into a MESH again without making any change, you will have a fixed bounding sphere. I'm working on a tool that can fix bounding spheres on a large scale (fully recursive directory search) for the upcoming version.