I wrote to Obie back in 2014 on Facebook and he was kind of enough to answer me about this very matter.

"Good question. Here's the deal: I went back to the original 2" analog session tapes to listen to everything after the tapes were baked and transferred to digital format. Those vocals are not on the original demo session tapes so they were added at some point to the original demo mixes. I looked at all the tape from those demo sessions and could not find them. It was cool hearing those rough demos again. Thanks, Obie"

Mystery solved.