IIF datediff/dateadd returns unexpected results

Hi,

I've got the code below and when summed into a blank textbox it returns a value of 564, yet when I run the query into an RS table and copy the results out to Excel and filter on 'First Time Success' I get a count of 596 rows in Excel. 

Can anyone see what's going on??

=IIf(DateDiff(dateinterval.Month,DateAdd(dateinterval.Month,-12,Today()), Fields!ChangeRequest_Complete.Value)>0 AND Fields!ChangeRequest_Outcome.Value="First Time Success", 1,0)

I'm trying to display in a chart how many change requests have had a first time success in the past 12 months.  I can;t see why the results differ as it's the same query/calculation running both times.