The development team say:
Two things going on here:
1. Ifc doesn't know the difference between a Revit area and a Revit room. So you can't roundtrip it without "cheating". Cheating in this sense means putting extra information in IFC that would only make sense to Revit.
2. The extra faces are just the 2D boundary plus the height to the next level. So we calculate that extrusion.
Cheers,
Jeremy