I am not sure if this can be done or not, i have searched quite a bit for a solution.
I want to calculate a variance between 2 columns of the same data for different periods in my pivot table. I am trying to calculate the variance between 2021 sales and 2020 sales inside the pivot table. in Office 2013
For Office 2013 that's if add data to data model; install Power Pivot add-in and add explicit DAX measures which calculate Sales, Sales for same period previous year and their difference, when create PivotTable taking data model as the source.
Just an idea, ready to use pattern very depends on concrete data plus 2013 is bit old, it may be compatibility issues if create in later version.