Your application is correct but for the first month to be displayed you need the total, in your situation from January to March.
Your underlying data does not include January or February:
If you fix this issue in your underlying datasource you will be able to calculate the rolling sum for the first two months as separate entities instead of null.
I went back and updated the month end values. The first 2 months still don't sum properly unless I have the first two months in my date range selection and then hide them from the display.
The date range selection is not simply a 'this is what I want to see' It is
a 'process this data'. So if you remove the first 2 months from the data
range selector you are actually removing them from calculations in the
view. What you may want to look at is the 'hide' instead of using a filter
and you would be able to make a calculated field that removes the first two
months from sight but not from how they effect calculations.