At some times, your red activities counter in the Activities tab may show a smaller number than the List View of your Activities tab.
For example in the above screenshot, the activities counter shows four and the List View shows nine activities.
This is because the List View is filtered to show activities assigned to 'Everyone' in the company Pipedrive account.
For reference, the List View of the Activities tab shows all the activities in the company that are visible to you.
If you'd like to see a list containing only your own activities, simply adjust the filter to your own name.