Sometimes the developer who setup the original design was "nice" enough to put fields that are related in tables but without actually creating foreign keys. So it's possible that CompanyID is related to revenue id without a foreign key existing. Or CompanyID may be related (without a foreign key) to a field in one of the other tables that is connected to revenue id.

If you find that to be the case, you may want to consider putting the foreign key(s) in place to make your life and the next guys easier going forward. 😉