How does RVM pick where to find the gem

I noticed that rvm has more than one directory for saving 2.3.0 gems.

/usr/share/rvm/gems/ruby-2.3.0/gems

/usr/share/rvm/rubies/ruby-2.3.0/lib/ruby/gems/2.3.0/gems

I want to understand how RVM decides which directory to use. I have this gem rspec_junit_formatter that does not work with bundler newer than 1.12.5 I get different behavior when the rspec task is defined in lib/tasks/rspec.rake file as opposed to the rspec task defined in a Rakefile in ruby non-rails projects.

In rails rvm picks bundler from the rvm/gems directory. In plain ruby rvm picks bundler from the rubies directory. How come?