What changes? For a start, the more people that are involved, the less each knows of the situation. A single individual being bullied is aware of each incidence and what it going on at all times. A group doesn't - witness mob justice as a clear counterpoint. A group is highly susceptible to hearsay and misdirection.

As for why it's unfair - if we expect the judicial system to be fair when it acts on our collective behalves, it is dissonant to not expect other group action to also be fair in how it's meted out.