Summary Sales Report: previous and current sales changes

[john123]

I have a slight consfusion regarding Calculating Sales Changes in list report that displays summary sales changes for each product in a store.

todays_sales:
if ([Date] = ?Today Date?) then ([Sales]) else (0)

yesterdays_sales:
if ([Date] = ?Yesterday Date?) then ([Sales]) else (0)

change_in_sale:
([todays_sales]-[yesterdays_sales])/[yesterdays_sales]

Grouping is done on Product_Id.  All of these data items have set the "Aggregate Function" to be calculated and "Rollup Aggregate Function" to be calculated. 

Sample data:

Product   Date                   sales
product A   Aug 29 2010   10
product A   Aug 29 2010   5
product A   Aug 28 2010   6
product A   Aug 28 2010   8

If I calculate manually, I should get:

chagne_in_sales =((10+5)-(6+8))/(6+8) =   0.071428571

Some reason all the change_in_sales for each product is either 0 or -1. 
I am not sure why this is happening.  I am not sure if I am doing it the right way when displaying change_in_sales in Summary level.  Can someone help with this simple issue I am having.