Hi Andy,
They are not exactly linked, more like one in the same. The Customer table is actually just a view on the larger Entity table. The Entity table contains all of your Customers, Leads, Vendors, Employees and Other Name records. When you look at the Customer table, you are really just looking at a filtered Entity table. This is why the Entity.RecordID and the Customer.RecordID are the same. It is the same record in the table, and therefore the same recordID.
Hope this helps,
- Adam