Detail level and Summary level total are not same for List or Cross tab report

[jd]

I have simple report studio report with just 3 columns Region, Location and Counts.

when I do totals for list or Cross tab report with Region by Location, report is showing detail counts for each location and correct totals for the report.

same report if I take as a summary level only Reion and totals, it not showing correct results.

ex: Total with detail level is        5,008,280.04
      Total only summary level  is  5,004,694.04(this is not correct count)

any idea why summary level is not showing corrct results.

Thanks in advance.