3DS anomalies
I did your tests and this is definitely a bug. You don't even have to move the
vertices on the top face of the box. Create a normal box, then select the top
vertices then do a Modify/Vertex/Align in the top view and the vertices will be
aligned in the y construction plane but the x and z values will be incremented
by, in my case +0.17.
DaviD "Modify/Vertex/Align (…and skew a bit)" GouID