Your problem is the double FULL OUTER JOIN as you can see if you use SELECT * to show all rows and columns.

Here's an alternative, but you should reevaluate the need of the full joins.

SELECT COALESCE(TPHY.HCPCS, TOUT.HCPCS, TDME.HCPCS) AS HCPCS

, COALESCE(MAX(UNITS_PHY), -1) AS UNITS_PHY

, COALESCE(MAX(UNITS_OUT), -1) AS UNITS_OUT

, COALESCE(MAX(UNITS_DME), -1) AS UNITS_DME

FROM MUES_PHY_2010_07 TPHY

FULL JOIN MUES_OUT_2010_07 TOUT ON TPHY.HCPCS = TOUT.HCPCS

FULL JOIN MUES_DME_2010_07 TDME ON TPHY.HCPCS = TDME.HCPCS

GROUP BY COALESCE(TPHY.HCPCS, TOUT.HCPCS, TDME.HCPCS)