Why does EXISTS return tuples with some nulls included

Question

I must be misinterpreting the function EXISTS.
Why does the following return lots of customers with null revenues?
What function should I be using or am I right to use EXISTS but need to use it differently?

WITH SET [CustomerSet] AS
    EXISTS( 
    [Customer].[Customer].MEMBERS,
        (
        [Date].[Date - Calendar Month].[Calendar Month].&[201312],
        [Measures].[Revenues])
        )
select 
   [CustomerSet] on columns
from [ourCube]

Answer 1

Ok - EXISTS basically mimics auto-exists behaviour without having to include two levels of a cross-join in the resulting cellset. If the two arguments are not from the same dimension then auto-exist behaviour does not occur - this is the reason for the failure of my original script. There is an optional third argument, which is very useful in this situation, and creates a context using a chosen measure group for auto-exist behaviour:

WITH SET [CustomerSet] AS
    EXISTS( 
    [Customer].[Customer].MEMBERS,
    [Date].[Date - Calendar Month].[Calendar Month].&[201312],
    "Revenues Measure Group"
    )
SELECT 
   [CustomerSet] ON COLUMNS
FROM [ourCube]