One trick is that you use CalculatedColumn to make an (A-B) column first and then you pipe data through Pivot process with "sum" of (A-B) column. It will give you the desired result.

The CalculatedColumn help you to create new column resulted from calculate from other. Here is its documentation. So basically you create new column to hold the result of (A-B).

        ->pipe(new CalculatedColumn(array(
            "a_minus_b"=>"{a}-{b}",
        )))

And then you pipe to the Pivot process:

        ->pipe(new Pivot(array(
            "dimensions" => array(
                "row" => "group_column, sub_group_column",
            ),
            "aggregates"=>array(
                "sum" => "a_minus_b",
            )
        )))

Here is the documentation of Pivot process.

Hope that helps.