Yes you are right. But if there are two sites one username is holding by one person on one site and the same username is holding by another person on different site then how can a person use the same usernames on both sites?.
User1: Bharath Reddy is holding vasireddybharath on some site lets say www.xyz.com
User2: Ramesh is holding same vasireddybharath username on some other site lets say www.zyx.com
Then what will happen?. Is the cards unique worldwide taking considering all the sites?.