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Cumulative sum not working correctly?

I have a running calculation for a count distinct on a number of items created per month. If I understood this correctly, this should calculate the count distinct for each month and when the running sum is added, it should add the values there were previously, so the difference between two months, should be the count distinct of items created in that month. However, this is not what is happening. Am I misinterpreting the way this works? 

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