Ok.
The stuff in the Atlas covers stuff up through HoME IX. In volume IV
Tolkien drew a ruff
map of the whole world, not just
Beleriand, and this is were the info for the full world
maps was gotten from.
The five
mountain ranges at that time were the Iron Mtns. in the North, the Blue and Grey Mtns. in the West and the Red and Yellow Mtns. in
the East.
When Tolkien wrote The
Hobbit he reused the name Grey Mtns., but at this time The Hobbit was not part of The Sil legends.
When he made the map for LOTR be the land East of the Blue Mtns, he made
the Grey Mtns. from The Hobbit to be the remains of the Iron Mtns. that were not destroyed during the
War of Wrath.
The map in volume IV no longer fit into the rest of the world and was set to the side and probaly forgot about.
Still I really like these additions to the Atlas and have no complaints about them being put in there.
But check out HoME IV The Shaping of
Middle-earth.