This is likely because the people you added are already in this Campaign/Event.  What it does is give them the member status of Sent (or whatever you set it to when you are adding the people in the report) just like if you were to add people by going through the Manage Members button. 

It doesn't create an Opportunity record.  If you click the Invitation List link in your Campaign, you will see the people that are "invited" to your event.  This doesn't mean that they have paid or have officially registered. 

As mentioned in the User Guide (Managing Invitees and Invitation Status), the Managing Members and Invitation List is a tool people can use but aren't required to use to manage who they invited to an event.  Some people may find it easier to just track this information elsewhere or not at all and choose just to track payments and actual registrations using the Opportunity record.