Hey Brooke,
I exported my own report and looked into several campaigns with a default status and it appears that they are campaings that are created but never run/activated. For example, I have template programs built with trigger campaigns for different program statuses. Depending on the event, I will not use some that I have created. When I look at my report, a large number of the default campaigns are these. I looked at others as well and it appears that my theory holds true for these as well.
Hope that helps!