Wordpress multisite/ A subscriber on my site?

So I am wondering about a user who just signed up today on my Wordpress multisite network. He signed up for his own blog which is great.

However, it says he is also a "subscriber" on my main site. I don't know how this can be because not one of my other members show as being a subscriber on my main site. They all just show themselves as admins of their own blog.

So how did someone create their own blog on my network but also become a subscriber to my main site? Upon registering on my site I completely took away the option for "just a username please" so Im perplexed how he did it when no one else has? Thanks