As an optimization to resource pack loading, everything in the relevant directory is compiled into this atlas, rather than only those sprites which are used by the game.
Flow and Guster are now mirrored on the reverse.[1]
Mipmapped versions
Minecraft also generates mipmapped versions of this atlas, for unknown reasons. They are not used by the game at all - only blocks placed in the world which use the json model system use mipmapped textures.[2]