The way war matchmaking is set up there can be up to 3 more player on one side. It allows the system more flexibility in finding match-ups, and is supposed to mean the side with the fewer players has stronger teams. Doesn’t always work that way in practice but many times it does.
Only ever happened to our alliance once. And I asked the same question.
Only to be told it’s not a bug.
But don’t worry too much. You can still defeat the opponents- we did
Good luck