TopCount not aggregating visible rows

[AKTMO]

I'm creating a crosstab report using dimensional data. It is a view of the top 25 suppliers by sales in 2013 year-to-date. Of these top 25, there is a side by side view of sales for the month and sales for year to date.
The topCount used is:
Top 25 = topCount ([Supplier Name],25,tuple([Sales],[2013]))

The first part of the report that lists the top 25 suppliers according to 2013 sales is returning the results I want. Which is the same set of 25 suppliers for both the month and year-to-date sales values.

The problem is with the summary. The year-to-date is correctly summing the top 25 according to 2013 sales, but the month is summing the top 25 according to month-to-date sales, which is a different set of suppliers. Since the top 25 according to monthly sales is different then the top 25 according to 2013 year-to-date.
The summary expression = aggregate(currentMeasure within set [Top 25])

Example using Top 3:
Supplier  Month-Sales   2013-Sales
A                16                     45
B                12                     43
C                10                     35
Sum            43                    123

Month-Sales Sum should be 38, but supplier D has 15 in month sales and only 32 in 2013 sales. So supplier D doesn't show since the Top 3 is according to 2013 sales.

Thank you