multiselect result changes while add additional dimension to filter?

Every Query I introduce with multiselect to restrict the my rows works only as long as I do not add a second dimension to the filter fields ?

My Query (derived from Management Studio) - if you run it once like this and once with the comment line enabled the count of columns returned change - what I really did not understand, because only the values should change (different slice)?

Could anyone explain this to me? or point me to a location with detailed explanation? (I have checked http://www.mosha.com/msolap/articles/mdxmultiselectcalcs.htm this but it does not hit specially my question)

WITH
MEMBER [Sales Territory].[Sales Territory Group].[{2120AF0D-0FD5-4B1F-8562-51CCF95CE627}Pivot8Hier164MultiFilter__] AS 'AGGREGATE({ [Sales Territory].[Sales Territory Group].&[North America], [Sales Territory].[Sales Territory Group].&[Pacific] })'
SET [{2120AF0D-0FD5-4B1F-8562-51CCF95CE627}Pivot8Axis1Set0] AS '{ { [Sales Territory].[Sales Territory].[All Sales Territories] }, AddCalculatedMembers([Sales Territory].[Sales Territory].[Group].MEMBERS)}'
SELECT
[{2120AF0D-0FD5-4B1F-8562-51CCF95CE627}Pivot8Axis1Set0] DIMENSION PROPERTIES MEMBER_NAME, PARENT_UNIQUE_NAME , [Sales Territory].[Sales Territory].[Country].[Sales Territory Group] ON COLUMNS
FROM [Adventure Works]
WHERE
(
[{2120AF0D-0FD5-4B1F-8562-51CCF95CE627}Pivot8Hier164MultiFilter__]
// , [Sales Channel].[Sales Channel].&[Internet]
)
CELL PROPERTIES VALUE, FORMATTED_VALUE, FORE_COLOR, BACK_COLOR

Noone with an explanation or hint to further documentation ?

Who is it possible that an aditional slice of an independ dimension changes the rows / columns if not filter or empty is used ?

HANNES