Actual - Previous Year not summing correctly

Hi,

I've tried two different MDX expressions to return the Actual GSV for this period last year.  They both work for individual periods but they don't sum correctly.

Ie if you review the results for several periods in Excel then as long as the periods appear across or down the page the results are correct. However if Period is the Page Grouping (the Filter) then the result is wrong.

The first expression doesn't return a total at all:

Create Member CurrentCube.[Measures].[Actual - Gross Sales PY]
As 
([Measures].[Actual - Gross Sales], 
ParallelPeriod(
[Date].[Period Hierarchy].[Fin Yr],1,
[Date].[Period Hierarchy].currentmember)),
NON_EMPTY_BEHAVIOR = { [Actual - Gross Sales] }, 
VISIBLE = 1 ,  ASSOCIATED_MEASURE_GROUP = 'Monthly Sales Forecast'; 

This version sums up all periods not just the two I've selected:

Create Member CurrentCube.[Measures].[Actual - Gross Sales PY expt]
As 
 Sum(EXISTING [Date].[Period Hierarchy].[Yyyy Period].MEMBERS,  
 ( ParallelPeriod( [Date].[Period Hierarchy].[Fin Yr],  
                   1,  
                   [Date].[Period Hierarchy].currentmember  
                 )  
 ,[Measures].[Actual - Gross Sales])),
NON_EMPTY_BEHAVIOR = { [Actual - Gross Sales] }, 
VISIBLE = 1 ,  ASSOCIATED_MEASURE_GROUP = 'Monthly Sales Forecast'; 

Any help would be much appreciated!