The usage of EXISTING keywork in MDX query

There's a demo from "MDX Step by Step 2008", it introduce the usage of 'EXISTING' to provide the context for set in calculated member.

WITH
MEMBER [Measures].[Number of Products] AS
COUNT(
       [Product].[Product].[Product].Members
) 
SELECT
{
            ([Measures].[Reseller Sales Amount]),
            ([Measures].[Number of Products])
} ON COLUMNS,
{[Product].[Category].Members} ON ROWS
FROM [Step-by-Step]
GO
WITH
MEMBER [Measures].[Number of Products] AS
COUNT(
       EXISTING [Product].[Product].[Product].Members
) 
SELECT
{
            ([Measures].[Reseller Sales Amount]),
            ([Measures].[Number of Products])
} ON COLUMNS,
{[Product].[Category].Members} ON ROWS
FROM [Step-by-Step]

Two outputs - the second output is correct.

Now I try another demo to get the last date with data, but I found there's no any difference when using 'EXISTING' or not.

WITH MEMBER [Measures].[Last Date]
AS
	 MAX({[Date].[Date].[Date].MEMBERS},
		 IIF(
				([Measures].[Internet Order Count]) = 0,
				NULL,
				[Date].[Date].CurrentMember.MemberValue
			)
		)
SELECT
{[Measures].[Internet Order Count],[Measures].[Last Date]} ON 0,
{[Promotion].[Promotion].MEMBERS} ON 1
FROM [Adventure Works]
GO
WITH MEMBER [Measures].[Last Date]
AS
	 MAX(EXISTING{[Date].[Date].[Date].MEMBERS},
		 IIF(
				([Measures].[Internet Order Count]) = 0,
				NULL,
				[Date].[Date].CurrentMember.MemberValue
			)
		)
SELECT
{[Measures].[Internet Order Count],[Measures].[Last Date]} ON 0,
{[Promotion].[Promotion].MEMBERS} ON 1
FROM [Adventure Works]

The outputs are same.

My expected result for last date in first query which don't have 'EXISTING' should be always 7/31/2008, because there's no context for Date member, so it always return the MAX date with data.

 MAX({[Date].[Date].[Date].MEMBERS},
		 IIF(
				([Measures].[Internet Order Count]) = 0,
				NULL,
				[Date].[Date].CurrentMember.MemberValue
			)
		)

Thanks!

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