I see Borg designations more like positions in a collectible set.
Like, how a set of action figures may say "Janeway: 1 of 10, Chakotay: 2 of 10", etc.
A borg unit is like a squad, a group designed to carry out a specific purpose. There are nine Borg required to perform that function, and Seven's role in that happens to be delegated the seventh role.