The Active tab on list view shows you only the incidents that meet your filter criteria that have not been cleared. For example, if you made these filter adjustments:
Only show Collisions and Accidents
Monitoring radius is 5 km
Only show incidents being reported live and up to 10 minutes ago
Then this means all incidents that meet these three criteria points will show in your Active tab.
The All tab will show all of your filtered posts and those that have been cleared (grey markers).