The logic used to count the number of active patients was badly incorrect. (It used "if the patient is active currently and was born before the date then they are active on that date".)
This has been updated to the logic used in the KPI report - ie "A patient is taken to be active at a given date if they have a Date Created prior to or equal to the given date and either have their Active flag set, or have activities after the given date."
The fixed version has been placed in the resource library - see http://www.openvpms.org/customisation/practice-summary-report-updates
Thanks to Sam L for finding this error.