calculate previous month values return a negative column for last month

[MicheleGenerali]

Hi all 
I'm usin an external calendar table to make to make a MoM comparison with the previous year. Now i'm trying to calculate the variance based on prevoius month for the current year, but when i do a measure it adds a last column with the negative total value of my previous month

This is my Dax expression : 
Measure =

    VAR __PREV_MONTH =

        CALCULATE(

            SUM('SBU_Prodotto'[Incassi Danni AC]),

            DATEADD('DATE'[Date].[Date], -1, MONTH)

        )

    RETURN

            SUM('SBU_Prodotto'[Incassi Danni AC]) - __PREV_MONTH

How can i have a function for having only the "green columns" ?