The Past Due report will only show a number in the "Total" column for the cumulative amount of installments that are past due as of the date range specified on the General Tab of your report. Only gifts who were Past Due as of the date listed under "Past Due as of" will be included in this report. So, if a constituent has a pledge that still has a number of installments left to be made in the future, this will not be included on this report.
The Pledge Status report functions differently than the Past Due report in that it will look for all activity for the pledge in the date range specified on the General tab and any payments that were not made within that range will show up listed in the Pledge Balance.
A good way to verify why the totals are not matching is to run the two reports in Detail format, possibly limiting it to just one specific fund. It is then possible to view the constituents who show up in each report. By looking at the Gift Tab on one of the constituents whose totals are not matching, it is possible to verify that the balance not showing up on the Past Due report is due to the installments being scheduled in the future.