There is a known problem that sometimes arises when using Summary report definition. The COUNT does not report correctly but when drilling down the next level of the report is correct. For example the summary level of the report might show 58 computers of a particular operating system but when you drill down there are 65 computers listed, which is the correct number.
The issue usually surfaces when selecting data from multiple tables. To limit the data to the resource type selected, a view called vResourceEx is used. When fields are selected from the other tables the combining of the data from the view and the other tables is not done properly. It is simple to correct the problem with just a few steps using the Advanced edit button which opens the Report Query Builder.
Steps 4-7 are the steps required to fix this issue and can be used with any Summary report.