In order to comply with SMIv2 which governs the structure of SNMP information. If you reference section 7.7 of rfc 2578:
Objects which are both specified in the INDEX clause of a conceptual row and also columnar objects of the same conceptual row are termed auxiliary objects. The MAX-ACCESS clause for auxiliary objects is "not-accessible"
UnDP only displays the "non-index" objects. SMIv2 expects the SNMP manager to determine the values of the index objects - which are also columnar objects - from the returns from the snmpwalk of the table. UnDP expects the objects to be at least read-only which violates rfc 2578. Right now NPM has no way of properly display any tables that include auxiliary objects.
Please refer to case 995197 for other details.