Implicit Aggregate in pivot table ?

Question

I am using Excel to query an OLAP SSAS cube. Without any filter, the pivot table I have is: 
Values
SCM Lines Count
94741
OTR_Net
87.28%
OTD_Net
95.00%
I need to consolidate these metrics on the first complete months of the year. So I create a named set. The corresponding code in the MDX query that Excel generates is:
SET [YTD] as VisualTotals({[Delivery Date].[Period Hiearchy].[All],YTD(StrToMember("[Delivery Date].[Period Hiearchy].[Month].&"+
FORMAT(Now(),"[yyyyMM01]")+ ".PrevMember"))})
When I drag and drop this set to the columns area of the pivot table, I get this:
Values
2014-01
2014-02
Grand Total
SCM Lines Count
3715
3608
7323
OTR_Net
81.77%
84.77%
83.24%
OTD_Net
91.66%
94.33%
92.97%
So far so good, but I would like to remove the months and only keep the Grand Total. But you cannot move a set to the page filter area, so I tried the following workaround with the add-in "OLAP Pivot table extensions": I remove the set from the
columns area (the result changes back to the first table above), and add a calculated metric:
[Measures].[YTDCount] AS Aggregate([YTD],[Measures].[SCM Lines Count])
And - surprise -, I get the result below: The other metrics are also filtered on the [YTD] set, even though it is not explicitly set as a filter in the pivot table.
Values
SCM Lines Count
7323
OTR_Net
83.24%
OTD_Net
92.97%
YTDCount
14646
The final MDX query is:
WITH
SET [YTD] as VisualTotals({[Delivery Date].[Period Hiearchy].[All],YTD(StrToMember("[Delivery Date].[Period Hiearchy].[Month].&"+
FORMAT(Now(),"[yyyyMM01]")+ ".PrevMember"))})
MEMBER [Measures].[YTDCount] as AGGREGATE([YTD],[Measures].[SCM Lines Count])
SELECT {[Measures].[SCM Lines Count],[Measures].[OTR_Net],[Measures].[OTD_Net]} DIMENSION PROPERTIES PARENT_UNIQUE_NAME,HIERARCHY_UNIQUE_NAME ON COLUMNS FROM [METRICS] CELL PROPERTIES VALUE, FORMAT_STRING, LANGUAGE, BACK_COLOR, FORE_COLOR, FONT_FLAGS
Even if I unselect my custom metric YTDCount (which gives twice the correct result btw), the result for the other metrics remains unchanged. The final pivot table is indeed what I wanted, but I do not want to use what seems a dirty hidden trick.
Can anyone explain why the custom metrics also impacts other measures ?
Would anyone have a proposal for a set definition that would generates either only one column or a hierarchy where I could hide the month level. ?
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Answer 1

I finally found the answer: create a calculated member instead of a Named set. I had not realized that it could also be done with the add-in.
In the pivot table, I have dragged the Delivery date dimension to the page filter area. I can then select the member "YTD", and get exactly what I wanted.