The "Open" category on the summary screen is based on the meta status "Open". It includes multiple statuses; Open, Fixed, and any other statuses that are defined as equivalent to Open by the tracker admin for your project.
For example, suppose the Tracker Summary block shows 21 Open artifacts. However, when filtering on the "Open" status, only five artifacts are displayed.
If you filter by "All Open" you will see all 21 issues with statuses defined as Open. If you filter specifically by "Open" you will see only the artifacts that specifically have the status "Open".