Rails 3 routing for namespaced controllers


I need to use legacy-style routing. And it's namespaced.

According to http://edgeguides.rubyonrails.org/routing.html, it should
be like the following.

match ':controller(/:action(/:id))', :controller => /admin\/[^\/]+/

/admin/c1/a1 works fine.

But the problem is that, if I have /c2/a2 (note it's not namespaced with
admin) /admin/c2/a2 also works even if I don't have Admin::C2Controller.
So /admin/c2/a2 is mapped to c2#a2.

This might not be a big problem but I feel it's weird.
What do you think?