class variable mystery

gistfile1.rb

#code.rb

class Resource
end

module SomeModule
  class Consumer
    def singleton_resource
      @@sr ||= Resource.new
    end
  end
end

#spec.rb

require 'rspec'
require_relative 'code'

describe SomeModule::Consumer do
  it 'should init resource once' do
    res = double('resource')
    Resource.should_receive(:new).once.and_return(res)

    SomeModule::Consumer.new.singleton_resource 
    SomeModule::Consumer.new.singleton_resource 
  end
end