I have had a look at your model and calculations and there are a couple of points for you to consider.
Currently your Fact Table ( All Recorded Donation Table) has 4460 distinct values for ID however your dimension (Former And Current Staff) has 1077 distinct values for Donor ID.
This Doesn’t seem correct as the dimension should contain All distinct values. So something you should have a look at.
In your current model the calculation you have written is looking at the 4460 ID values and is therefore returning the correct result.
I can see in your distinct count of donors you have done this over AllRecordedDonationsTable[DonorID] which returns the 1678.
I amended the formula to look at this value and think this is returning what you are after.
If you need any further explanation or help do not hesitate to shout.
Donations _HA.pbix (601.0 KB)