Webi max function returning wrong value.

Hi,

I am trying to fetch max value based on some filter in formula bar in Webi report, but its returning wrong value, here is what I am trying to do.

=Max([Date Key]) 
Output is as expected:20140914

But when I add filter to it, it starts giving value in multiples, I tried all possible permutation/combination in below, but it didn't work or it returns #MULTIPLEVALUE.

=Max([Date Key])  Where ([Date Key]<=[vWeekStart]) 
Output: 11515471891, similar different values at each row also appears.

Variable: 

[vWeekStart] = ToNumber(FormatDate(RelativeDate(Max([Full Date]) in Report;-7);"yyyyMMdd"))
Output:20140907

Please guide me if I missed anything.

Thanks