You created a rollup of active announcements from all the Announcement lists in your site collection. You see 10 announcements listed, but another user sees only 8 announcements. Why is this?
List Rollups are security trimmed. Users will see only those items that they would be able to browse or search for manually.
In this specific case, the user who sees only 8 announcements may not have permission to see all the existing announcement lists or announcements.