Why would `rvm install ruby-2.1.0` install preview1?

I just tried to install Ruby 2.1, and instead got 2.1 Preview 1. This seems crazy to me. 2.1 is out. Why would RVM assume that when I say rvm install ruby-2.1.0 that I really mean rvm install ruby-2.1.0-preview1? Why would it not match the exact version I specified, instead of one which begins with that substring?