If someone else invited his friend b4 he did, he would go on another server..
Let's say that you'r on server 1 and im on server 2
You invite a guy to your server, after 2 days i invite the same guy to my server.. he would automaticlly become my referral if he joins the game in the next 7 days but he would be on your server..the first one to invite gets the referral on his server.
Simply way to find out is "Friend's fixtures" If you'r friend is in there then it's on your server.