SSAS How to calculate Standard Deviation
The StDev function does operate at a rollup grain per the first parameter you pass in. If you always want to calculate the StDev at the lowest level then you can write some mathematically equivalent measures and avoid the StDev MDX function. This approach is described here. Let me know if that doesn't make sense. The MDX would be:

(([Measures].[Sum of Square] - (([Measures].[Sum of Values]^2)/[Measures].[Count of Values]))/[Measures].[Count of Values])^0.5


See Peter's answer for a full explanation of the steps he took. 

Also, if you do stick with the MDX approach you are currently using you will want to fix the first parameter as follows. Your code is factoring the grand total into the calculation. This MDX does not:

STDDEVP( {[Data Source].[Data Source Name].[Data Source Name].MEMBERS},[Measures].[Total])