I was trying to understand the reports that piwik generate for a month and compare that with report generated for date range for the same month using date range selector. I used http://demo.piwik.org for this test. I selected a month for Jan 2015 using Period: Month and then Period: Date range and selected January 2015.
Attached is the partial report with the discrepancy on the number of unique visitor and unique returning visitor. When trying to sum up the numbers for the whole month seems to be far off from the month aggregate data.
Please correct me if I have missed something here.
Piwik_DR_UV_URV.png is report exported using Period: Date Range [1st Jan 2015 to 31st Jan 2015]
Piwik_MONTH_UV_URV.png is report exported using Period: Month [Jan 2015]