Index Called PrimaryKey

Hi,
To make a long and tedious story short:
Does anyone know why Access (2003) creates an index called PrimaryKey and an index called User_ID when I'm in the table design view and ask it to make the User_ID field the primary key?

After years of database development I've only just noticed this. Not normally not a problem but for this:
Use ADOX to find the field name of the primay key for a table. I took the code right out of a Access 2000 developers handbook. It creates a collection of the table indexes and steps through them one at a time checking if the PrimaryKey property is true. It hits the index called PrimaryKey and says 'yes it's true, this is the PK', I then go on to use the index.name property in an SQL Select to create an ADO recordset. The only thing is that the index called PrimaryKey is not a field name and the field name User_ID is not a PK :eek:
I go into the indexes list in table design and delete and re-create the indexes and PKs as I want and the code works fine.

So, again, why does Access do this? I'm so confused! :confused: