Data Aggregation Issue

This issue occurs because your DAX measure might not be correctly handling the slicer selections and is summing up instead of averaging based on the unique donor count. Try doing somehing like this.

Frequency = 
VAR UniqueDonors = DISTINCTCOUNT( Donations[DonorID] ) 
RETURN 
DIVIDE( SUM( Donations[Amount] ), UniqueDonors, 0 )

If your frequency metric is in a table that is affected by slicers, wrap the calculation in CALCULATE to ensure it respects slicer selections.

Frequency = 
VAR UniqueDonors = CALCULATE( DISTINCTCOUNT( Donations[DonorID] ), ALLSELECTED(Donations[Ethnicity]) )
RETURN 
DIVIDE( SUM( Donations[Amount] ), UniqueDonors, 0 )

This will ensure that when multiple are selected, the correct unique donor count is used rather than incorrectly summing across selections.

Let me know if this helps!