GitHub organizations are actually free (although, of course, there are paid plans for private repos and enterprise features): https://help.github.com/articles/creating-a-new-organization... It's often a way that people group collections of related subprojects of an open source project, e.g. Babel is an org, with babel/babel being the primary repo: https://github.com/babel

People who make an org rather than just a project are probably more serious about it, which explains the higher activity rate, but it's still just "someone on the internet tried out an idea".