MERGE in SQL Server

Techies--

Will someone explain to me why this MERGE statement is setting the RecordIsDeleted flag to 1 on every row? Obviously NOT MATCHED by SOURCE trips it-- but I thought the criteria for the MATCH is on the 2 keys under the ON condition--not every column after the ON, which is what it appears to be doing.  Any advice on how to correct this?

MERGE dimension.CustomerSegment  AS TARGET

	-- SELECT ALL COLUMNS IN THE SOURCE ETL TABLE
	USING ( SELECT DS.SegmentKey
				 , DC.CustomerKey
				 , DCS.[MemberInternalKey]
				 , DCS.[BuyingUnitInternalKey]
				 , DCS.[SegmentInternalKey]
				 , DCS.[MemberDK]
				 , DCS.[BuyingUnitDK]
				 , DCS.[SegmentId]
				 , DCS.[SegmentDescription]
				 , DCS.[MatrixMemberId]
				 , DCS.[ClubInternalKey]
				 , DCS.[UpdatedDate]
				 , DCS.[StartDate]
				 , DCS.[EndDate]
				 , DCS.[AttachmentSourceId]
				 , DCS.[AttachmentSourceName] 
				 , DCS.[RecordFromDate]
				 , DCS.[RecordToDate]
				 , DCS.[RecordIsCurrent]
				 , DCS.[RecordisDeleted]        
			  FROM [etl].[CustomerSegment]		DCS
			 INNER JOIN dimension.Customer		 DC		ON DC.MemberDK				=	DCS.MemberDK
													   AND DC.BuyingUnitDK			=	DCS.BuyingUnitDK
			 INNER JOIN dimension.Segment		 DS		ON DS.SegmentInternalKey	=	DCS.SegmentInternalKey

          ) AS Source
                 ( SegmentKey
				 , CustomerKey
				 , [MemberInternalKey]
				 , [BuyingUnitInternalKey]
				 , [SegmentInternalKey]
				 , [MemberDK]
				 , [BuyingUnitDK]
				 , [SegmentId]
				 , [SegmentDescription]
				 , [MatrixMemberId]
				 , [ClubInternalKey]
				 , [UpdatedDate]
				 , [StartDate]
				 , [EndDate]
				 , [AttachmentSourceId]
				 , [AttachmentSourceName]
				 , [RecordFromDate]
				 , [RecordToDate]
				 , [RecordIsCurrent]
				 , [RecordisDeleted]
				 )
		ON (    Source.CustomerKey		= Target.CustomerKey		-- Key will be customer, segment, and segment start date.	
		         AND Source.SegmentKey	= Target.SegmentKey			-- Remember that segments can come and go
			
	

		   )

		-- NO MATCH IN TARGET MEANS NEW DATA - PERFORM AN INSERT
		WHEN NOT MATCHED BY TARGET THEN
	    INSERT (   [SegmentKey]
				  ,[CustomerKey]
				  ,[UpdatedDate]
				  ,[StartDate]
				  ,[EndDate]
				  ,[AttachmentSourceId]
				  ,[AttachmentSource]
				  ,[RecordFromDate]
				  ,[RecordToDate]
				  ,[RecordIsCurrent]
				  ,[RecordisDeleted]
				  ,[InsertedBy]
				  ,[InsertedDate]
				  ,[ModifiedBy]
				  ,[ModifiedDate]
		      )
        VALUES
             (   Source.[SegmentKey]
				,Source.[CustomerKey]
				,Source.[UpdatedDate]
				,Source.[StartDate]
				,Source.[EndDate]
				,Source.[AttachmentSourceId]
				,Source.[AttachmentSourceName]
				,Source.[RecordFromDate]
				,Source.[RecordToDate]
				,Source.[RecordIsCurrent]
				,Source.[RecordisDeleted]
			    ,@ExecutedBy						-- InsertedBy
			    ,GetDate()						    -- InsertedDate
			    ,@ExecutedBy						-- ModifiedBy
			    ,GetDate()						    -- ModifiedDate
		     )
		-- NO MATCH IN SOURCE MEANS WHAT SEGMENT DID EXIST FOR MEMBER IS NO LONGER CURRENT
		-- *Note: Because deleting/fragmenting indexes can be a performance killer, the row wil
		--        be flagged as RecordIsDeleted to TRUE.

		WHEN NOT MATCHED BY SOURCE THEN
		 UPDATE	
		   SET RecordIsDeleted		=	1   -- Set for future delete/reindex maint job.

		WHEN MATCHED			
         THEN UPDATE	
			SET Updateddate				=	Source.[Updateddate]		
			,StartDate					=	Source.[StartDate]			
			,EndDate					=	Source.[EndDate]
			,AttachmentSourceId			=	Source.[AttachmentSourceId]
			,AttachmentSource			=	Source.[AttachmentSourceName]
			,RecordToDate				=	Source.[RecordToDate]
			,[RecordIsCurrent]			=	Source.[RecordIsCurrent]
			,[RecordIsDeleted]			=	Source.[RecordIsDeleted]  
			,ModifiedBy					=	@ExecutedBy						-- ModifiedBy
			,ModifiedDate				=	GetDate()						--  ModifiedDate	


 ; -- END OF THE MERGE STATEMENT

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