As stated above, it's all due to which stage of the demoing process the song stems from. If Jon knew that it was only an early "to-go-from" recording, he'd settle for a guide vocal rather than going full force with his voice (remember that he always pushed himself to the limit for the album recording, at least until the mid-2000's).

The sound of the demos themselves vary because some might've been regarded to be "just a demo" and others (like Postcards from the Wasteland, Why Aren't You Dead, The Radio Saved My Life Tonight, Wedding Day etc.) migh've been considered as a possible album cut at one point or another. There is a difference in effort (also from the producer's side) when a track is getting mixed for an upcoming release. Certain effects for ambiance get mixed in, instruments might get doubled, lead singers could go in and back their own vocals (like on Blaze of Glory). That's why a demo like the ones stated above sound very good while, e.g., the published batch of NJ outtakes aren't that good. Back then the record company pulled the plug on Join going for a double-album and therefore work on some tunes was abandoned before they got to the final stage. What you hear there are basically the raw instruments and vocals mixed together, hence the more "unpolished" sound.