well how I can expain...
Some of These sprites are the beta (or even alpha) versions of the sprites, they never got released by gravity, because they got changed soon after get started. Maybe some of them got released with rebirth update (just like said, maybe if you got luck you'll find a instalation disc with it).
Just like the new jRO Third job sprites, some of them got some changes before the release, and the previous beta version of the alt. sprite does not exist anymore, because the old version is replaced to the new during the spriting process.
Most of them are just Concepts