I have a theory as well, a huge theory, I know it's hard to believe, but my theory is that Michael sang all these songs, to varying levels of finality, some in demo/rough shape, some that he was further along with (but none final), and then producers came in and used these vocals to create songs. Michael's voice, producer's visions. They looped some vocals (repeating the same verses in some cases), they used varying degrees of processing (listen to the weird processing in Best of Joy), they used different equipment in their studios, different mixing equipment, and different levels of fidelity. And then they put all these songs on a single CD. Now guess why they would sound different. It's not too difficult to figure out. They're all Michael, just presented differently, by different producers.