When selecting Additional Addressees/Salutations on the fields to include tab, when you configure multiple additional Addressees/Salutations, the Addressees/Salutations are listed as:

    Additional Salutation - 1

    Additional Salutation - 2

    Additional Salutation - 3

    Additional Salutation - 4
    In the merged document, the additional salutations are listed as:

      Additional Salutation

      Additional Salutation_1

      Additional Salutation_2

      Additional Salutation_3

      So, when you select Additional Salutation_3 on the letter, it is actually the fourth additional salutation created.