What if the band didn't want it to be considered part of the discography, but the name got stuck on it anyway? I'm thinking the Sabbath albums when it was more Tony Iommi And Friends, and he just wanted to do his own thing with a solo project? Sure, you've got Never Say Die and Technical Ecstasy stuck between eight great studio albums, so they wouldn't quite count for consistency, but after Mob Rules I'm pretty sure everyone except the label knew that Sabbath was done making new material.