Michael has LOTS of unreleased songs.. the Estate will release new MJ stuff untill 2017 (due to the contract they signed with Katherine). He was always writing/recording... even when he wasn't releasing new albums. He was always creating..
With the album Michael, they put on songs that MJ recorded before, like ryanscupcake39 told you, some songs were unfinished by Michael.. and they "finished" the work in his place. :(
Btw, the last album of MJ, Bad 25, is awesome! and all the new tracks are original, no changes made!! :)
