I am not really sure how the screenshot relates to the expression, as there is no count column in the table.
Just for reference, have you compared by doing the selection manually?
If you have the expression without the set expression like Count(DISTINCT INVOICE#) and then manually select SalesMonth='Oct' and DateType='Invoice' I would expect it to show you the same result. The point with this is that it might not be the set expression that is the issue, but perhaps the way your data is related.
An unfortunate join, synthetic key or field for multiple data islands could lead to that you get all possible combinations of data in the data set being aggregated.