The boxes shown in the relationship graph in Manage | Database | Relationships aren't really tables though filemaker calls them that.
You have two different objects on the two tabs. On the tables tab, you have data-source tables. These are the actual tables defined in the current file. On the relationships tab, you have table occurrences. They are labels that refer to a data-source table and often have the same name as the data-source table but there can be more than one table occurrence box for the same data-source table.
In order to link the same two data-source tables with different relationships, you often have to create some new boxes on this graph that refer to the same data-source table. Hover your mouse over the arrow in the upper left corner of one of these table occurences and the name of it's data source table will popup. You can also double-click one of these boxes to see the data source table. On the Tables tab, you'll see all table occurrences for a given data source table listed in the right hand column.
I realize this can be confusing for the new user. Feel free to keep asking questions.
Some people have commented that the article at the following link is hard to understand, but you might take a look at it and see if it helps: Table vs. Table Occurrence (Tutorial)