Ruby private weirdness

gistfile1.rb

class Foo
  private
  attr_accessor :foo  

  public

  def access_foo
    foo # => nil # as expected
    self.foo # => NoMethodError: private method...
  end
  
  def set_foo
    self.foo = 'bar' # => 'bar' # works just fine, presumably because `foo = 'bar' would just set a local variable...
  end
end

f = Foo.new

f.foo # => NoMethodError: private method `foo` ... # as expected
f.foo = 'foo' # => NoMethodError: private method `foo=` # as expected
f.set_foo # => 'bar' # works