Need Help to understand the piece of code

I am able to access all the instance variable like this ClassName.registered_robot_engine except default_robot_engine. why?

class Dog
  class<< self
    attr_accessor :registered_robot_engine

    def set_stuff
      @registered_robot_engine = 'hello'
      @default_robot_engine = 20
    end
  end
end

Dog.set_stuff
puts Dog.registered_robot_engine
puts Dog.default_robot_engine

--output:--
hello
1.rb:16:in `<main>': undefined method `default_robot_engine' for Dog:Class (NoMethodError)

The basic rule in ruby is that all instance variables are private by default, so unless you provide accessor methods for an instance variable you can't access it. In the example above, there are no accessor methods defined for @default_robot_engine, so it is inaccessible, while the other instance variable does have accessor methods defined for it, so it is accessible.

Both class_eval() and instance_eval() allow you to violate encapsulation and read or write private instance variables:

class Dog
  class <<self
    def set_stuff
      @registered_robot_engine = 'hello'
      @default_robot_engine = 20
    end

    def set_more_stuff
      class_eval do 
        @default_robot_engine = 100 
      end
    end
  end
end

Dog.set_stuff
Dog.set_more_stuff

puts Dog.class_eval{ @default_robot_engine }

--output:--
100

instance_variable_set() and instance_variable_get() allow you to do the same thing:

class Dog
  def initialize
    @name = "Rover"
  end
end

d = Dog.new
d.instance_variable_set(:@name, "John")
puts d.instance_variable_get(:@name)

--output:--
John

Secondly, it is hard to imagine why the programmer didn't use standard getter and setter, as in:

class << self
  attr_accessor :default_robot_engine
end

I would guess that the programmer is using someone else's module, and the programmer has decided to violate encapsulation, which ruby allows you to do. Some languages believe that although encapsulation is good, it shouldn't be strictly enforced. If for some reason a programmer wants to violate encapsulation, they should have the freedom to do so.