Wrong Subtotal when using Variable

Hello,

I get a wrong result in my subtotal when I utilize a variable.

 I use months as a filter.

I cannot see the connection.

Diff Actual Billing - Budget = 
Var DiffActualtoBudget = [Total Actual Billing] - [Total Budget]
RETURN
//DiffActualtoBudget
IF(
    HASONEVALUE(dimCalendar[MonthYear]),
    DiffActualtoBudget,
    SUMX(
        VALUES(
            dimCalendar[MonthYear]),
            DiffActualtoBudget))    

Please see screenshot. 

Thank you,

Tamir