dax expressions

First of all, I put your code in www.daxformatter.com to come up with the following:

YTD_Variance_Matrix =
CALCULATE (
    TOTALYTD (
        [MTD_Actuals_Matrix],
        V_FS_DIM_DATE[act_date]
    ) - TOTALYTD (
            [MTD_Budget_Matrix],
            V_FS_DIM_DATE[act_date]
        ),
    KPI_Finance_Matrix[Group] = "Revenue"
) & (
        CALCULATE (
            TOTALYTD (
                [MTD_Budget_Matrix],
                V_FS_DIM_DATE[act_date]
            ) - TOTALYTD (
                    [MTD_Actuals_Matrix],
                    V_FS_DIM_DATE[act_date]
                ),
            KPI_Finance_Matrix[Group] = "Operating Expenses"
        )
    )

You can see about halfway down you use the character "&".

This will concatenate the 2 otherwise numeric values into a single text string, probably not what you want.

Consider replacing that & with a + or a - depending on the arithmetic you need to perform.