I think that like many recording artists, lots of stuff gets made but doesnt get released. One recent example is Kylie Minogue's new Album 'X', over 30 songs were wrote and recorded but only a dozen or so make it to the final album, a few others make it to the B-SIDE of singles released, the rest go to the archives, and fans only hope they one day get released in the future! Im sure the Henson archives are full of stuff that have never been seen or made public, thankfully, for us this album did see the light of day!
It was never released in the UK but I managed to get one imported, Its a great album.
Chris