Using 4.6.10 currently, although aiming to update shortly, I've created a case summary report from the template that simply lists all cases that are either 'ongoing' or 'urgent'. This lists 10 cases. All good.
However, if I then use 'Cases' > 'Find Cases' and specify the exact same criteria (i.e. find all cases with the status 'ongoing' or 'urgent', it lists 14 cases.
Comparing the two results lists and looking for differences between the cases that might explain the difference in results, I can see none. We have only one case type, and all the cases as listed in the civicrm_case table carry the same case type id, they are all open and not deleted, the status id values marry up as I would expect, and I can see no differences that might lead to these four cases not being included in my report.
My assumption/conclusion is that the case summary report template is in some way broken in this version? As I say I'm about to update to the current 4.6.x version (4.6.14 at the time of writing), but wondering if others have had odd results with the Case Summary Report, or indeed the 'Find Cases' search facility?