Why does this happen?

class_methods.rb

class Thing
  class << self
    def public_method
      private_method
    end

    private
    def private_method
      "hello"
    end
  end
end

Thing.public_method # => "hello"
class Thing2
  class << self
    def public_method
      self.private_method
    end

    private
    def private_method
      "hello"
    end
  end
end

Thing2.public_method # => NoMethodError (private method `private_method' called for Thing2:Class)