Strange let behavior with threads

gistfile1.txt

class Foo; end

describe Foo do
  
  # expected behavior
  context 'when not testing with let' do

    it "uses the same Foo object inside threads" do
      foo = Foo.new
      obj_id = @foo.object_id
      Thread.new do
        foo.object_id.should eql(obj_id)
      end
    end

  end

  # unexpected behavior
  context 'when testing with let' do

    let(:foo) { Foo.new }

    it 'creates a new Foo object inside threads' do
      obj_id = foo.object_id
      Thread.new do
        foo.object_id.should_not eql(obj_id)
      end
    end

  end

end