I think because a commercial TOPO map in 1:25.000 or 50.000 scale has another DEM-resolution than the seperat Nüvi/Zumo-files. I would be nice to have a tool to extract this DEM's from Garmin and use it also in other programs (Global mapper, QuoVadis, ArcGIS,...).
Somebody here ( [Only registered and activated users can see links. ] ) did a great job and reworked the SRTM data for special areas to good DEM's.