Z Score calculating incorrectly

Hello,

I'm trying to calculate Z scores by Store with a Calendar Week Slicer. If I select a single week my z scores return correctly, however, if I default to All they inflate. 

Here's my formula: 

Sales Units Z Score = 
    var Mean = CALCULATE(SUMX(Values('Calendar'[Week]),[Avg Sales Units]),ALLEXCEPT('Reports DM_STORE','Reports DM_STORE'[Store Number]))
    var StandardDeviation = CALCULATE(STDEV.P('Reports FACT_SUMMARY'[SalesUnits]),ALLEXCEPT('Reports DM_STORE','Reports DM_STORE'[Store Number]))
return (SUM('Reports FACT_SUMMARY'[SalesUnits]) - Mean) / StandardDeviation

I tried altering hte STDEV.P formula to the STDEVX.P to iterate over the records, but it still returns the same Z Score when it's all said and done. 

Single Week Slicer Results:

StoreNumber: 1; Sales Units Z Score: 2

All Week Slicer Results: 

StoreNumber1; Sales Units Z Score: 98.64

Any idea how I can aggregate appropriately dependent on my slicer selection?

Any help would be appreciated!

Thanks,
Channing