I think if there's any meaning to the designations, it's just the order in which the drones are introduced to the group. The Borg don't have any apparent use for ranks. All drones are equal, but openings within a subgroup can be filled at any time.
Meaning, "Seven of Nine" would not have been less important than One through Six, or more important than Eight and Nine. That just happened to be the order in which those positions became filled.