If memory serves me correctly, I had a problem a while back, regarding the new map install not over-writing the old maps.
What had happened was that , if you load new maps onto the computer first, and then copy this across to the Garmin, it creates a differently named file, and this , having a different name, is obviously not over-written.
I queried this with garmin, as I was seeing past maps in the window where you tick which maps you want to use ( or not), eg 3D Maps, etc.
They instructed me to remove the file called -"Garminmapsup.img", and that on downloading new maps directly to the Garmin Zumo, ( ie not dragging file from the Mac across), the new maps file would then be correctly loaded, and would appear as
"Garminmap.prom.img"
When I subsequently checked in the Map info window again, the earlier maps had disappeared, and only the latest map versions showed.
Worked for me with Mac and Zumo 660.
Hope this helps, but I do agree, it should never have to be this complex!