Guard frequency (121.5) is different than "guard bands" in this context. Guard bands are, as was guessed, a frequency range at the edge of an allocation where n

Guard frequency (121.5) is different than "guard bands" in this context. Guard bands are, as was guessed, a frequency range at the edge of an allocation where no energy is /intentionally/ transmitted, and where received signal /should be/ disregarded. Basically it allows the reality of some frequency spillage to be accommodated between what would otherwise be adjacent frequency allocations.