You say “deactivate“ but Facebook does differentiate between deactivation and actual closing of an account. So if you merely deactivated it, then that would explain why your account was not actually deleted and it reactivated it for you.

> The biggest difference between deactivating and deleting a Facebook account is that deactivating your Facebook account gives you the flexibility to return whenever you wish, while deleting your account is a permanent action.

From what I remember, reactivation of a deactivated account merely requires logging back in. Which sounds like you did exactly that.