Let's clarify a few things here -
a) We don't know for sure if the setlists of both shows were the same
b) We don't know how much of the vocals (lead or backing) were overdubbed.
All we know is there were some backing vocal overdubs, and we can only confirm it 100% because two different versions of Now I'm Here have been released - one of which clearly had backing vocal overdubs (the Greatest Video Hits version seems to be purely the live version).
As for the overdubs being from the studio albums, it's entirely possible. On this song they do sound pretty similar.
"The more generous you are with your music, the more it comes back to you." -- Dan Lampinskihttp://www.queenlive.ca