[railstutorial.org] When to and not to use instance variable?

I am currently at chapter 8 of the Rails Tutorial. I am a little confused with [Listing 8.23](http://ruby.railstutorial.org/chapters/sign-in-sign-out?version=4.0#code-signed_in_p). Why does the `signed_in?` method not use `@current_user` but `current_user` instead? Thanks!