THE RPTR RECEIVED AND RESPONDED TO GPWS WHEN COMPLYING WITH DSCNT INSTRUCTION PREMATURELY ASSIGNED BY APCH CTL.

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ON THE BONVIL ARR TO RWY 28R AT PDX ON THE PDX 079 DEG RADIAL; 30 DME DECREASING TO 25 DME; PDX APCH ISSUED A NON DISCRETIONARY DSCNT FROM 5500 FT MSL TO 4100 FT MSL IN SOLID IFR WX. AT 21 DME THE GPWS 'PULL UP/TERRAIN' WARNING SOUNDED 4 TIMES. AN IMMEDIATE MAX PWR CLB WAS EXECUTED TO 7500 FT MSL. PDX WAS ADVISED ON RADIO. THEY RESPONDED THAT THE CLB WAS UNNECESSARY; 4100 FT IS A GOOD ALT. NO TFC CONFLICT OCCURRED. 1 WK LATER THE SAME ARR WAS FLOWN IN VMC WX. THE OFFENDING TERRAIN IS AT 21 DME. PDX APCH WAS QUESTIONED AS TO THE BEGINNING OF THE 4100 FT MVA; AND THEY RESPONDED WITH 18 DME. SO; IT APPEARS THAT THE PREVIOUS WK'S ENCOUNTER WAS A RESULT OF A CLRNC ISSUED THAT ANTICIPATED THE JET TO BE WITHIN 18 DME PRIOR TO REACHING THE 4100 FT ALT. THE MATH THEREFORE INDICATES AN ACTUAL MISS DISTANCE OF 600 FT. WHILE NOT FATAL; THIS IS TOO SMALL A MISS DISTANCE AND THE PRACTICE OF 'ANTICIPATING' BY CTLRS SHOULD BE REVIEWED. ALSO; WHY DOES PDX APCH WANT ACFT AT 4100 FT AT 18 MI -- 6000 FT SOUNDS BETTER TO ME.