The Category field is not filtered by date filter, as per product design. The Category column is not actually its own sub-level, but rather an additional informational column associated with site. In this context, the category column displays all categories that have ever been associated with a site in the database, independent of the associated user or date filters. This behavior optimizes query performance.
To create a report that applies filters to the Category column, the Category column needs to be added as its own sub-level (three-level report). This is a more resource-intensive report to run, which is why the unfiltered categories info field is provided in the simpler two-level site by user report. However, to see the categories filtered by user and date, a three-level report is required.