Change Fannie Mae Discriminating action Against Owners of Single-width Manufactured Homes

Per Fannie Mae Selling Guide: Fannie Mae Single Family dated 24 Sept 2013, Multi-width
manufactured homes can be located on an individual lot; but, single-width manufactured homes must be located in a Fannie Mae approval subdivision. Why is there a difference between a location for multi-width manufactured homes and a location for single-width manufactured homes?
Fannie Mae has a limit for manufactured home size of 'at least 12 feet wide and a minimum of
600 square feet of gross living area' that should apply to both type of manufactured homes. WHERE the homes can be located SHOULD be the same! There is no difference in the way the two types are built; only, the way that they are transported to the lot's location. HARP refinance denied because of Fannie Mae action