It seems to me all BLF/DSS keys on the SX-200 have this (pre-pended with DSS or BLF)... From the system documentation:
DSS Key labels are derived from the NAME field of the extension in CDE Form 09 (main form) and prepended with "DSS". The Name field is not editable when programming a DSS/BLF key but can be modified in the main form. For example, to edit a DSS Key label on Extension 123, change the Name associated with Extension 123 in CDE Form 09 (main form). If the name field is blank, phones will display "DSS 123"
Are you sure the other keys are BLF/DSS and not speed call keys?
Also note: To delete a DSS/BLF key on the Mitel 5330 or 5340 IP Phones, use the Superkey. Using the Settings key results in the error message "This key can only be programmed by your administrator."