Unita Fortior

My local football team, Carlisle United, has the Latin motto 'Unita
Fortior' i.e.'Stronger United'. Can anyone explain why the feminine
form is used in this context? In modern Latin based languages the
masculine form would be used by default where there in no noun. Why
is the masculine (or indeed the neuter form) not used in this context
in Latin?
Incidentally this topic was discussed some time ago on a supporters
message board and no-one could give an answer.
I hope this question isn't too trivial; there is a serious linguistic
question involved.

Thank you

David James