Product Reports off when interlapping year?

Hello,

Running on 1.5.6.4. Noticed something weird with the product reports.

My fiscal year is from July1-June30. When I pull a report for products sold, the sum of "Total" is different if I pull the data in 1 report (July1-June30 next year) or 2 reports (July1-Dec31 + Jan01-June30).

The single report has a larger total than the the merged dual. Is this expected? What could be the cause?

Thanks