I think some areas were never intended to allow house placement but due to bugs and in some instances OSI staff doing the placing, houses have been placed there anyway.
For instance on Pac there is a small one room house that sits atop a tower near the east Brit swamp, there are also several placed in east Brit near the fields.
Those are all old houses but there were recently some housing change due to a bug? that allowed houses to be placed at moongates? and in east Brit fields once again, but those newer houses were deleted by game staff or their owners.
Pretty sure you can trade old houses that currently exist in 'housing can not be created in this area zones'.
Once those old houses fall however, the no housing rules apply and houses no longer can be placed there.