In ST:TNG 523 "I, Borg" Picard meets Hugh. When Picard asks his Borg designation, Hugh responds that he is "Third of Five," using an ordinal number. On the other hand, in ST:VGR, Seven of Nine's designation is--well, that. It's a cardinal number, rather than the ordinal "Seventh of Nine." There are probably a bunch of other examples throughout the various series that I can't think of off the top of my head.
Is there an in-universe reason for the difference in designation? Or, for that matter, is there an out-of-universe explanation for the inconsistency?