Why does a Team Member show assigned by Unknown?

When you click on the 'Team' option in the left side menu of our website, you'll be directed to the Team Member page

Here, you can see who invited each team member by checking the 'Assigned By' column;

If the person who originally invited them is no longer a team member on that GreyMetrics account, the 'Assigned By' column will simply say 'unknown'