Are there good reasons for 'private' to work the way it does in Ruby?

It took me a while to understand how private methods work in Ruby, and it really strikes me as being very awkward. Does anyone know if there are good reasons for private methods to be handled the way they are? Is it just historic reasons? Or implementation reasons? Or are there good solid logical reasons (ie. semantic)?

For example:

class Person
  private
  attr_reader :weight
end

class Spy < Person
 private
  attr_accessor :code
 public
  def test
    code          #(1) OK: you can call a private method in self
    Spy.new.code  #(2) ERROR: cannot call a private method on any other object
    self.code     #(3) ERROR!!! cannot call a private method explicitly on 'self'
    code="xyz"    #(4) Ok, it runs, but it actually creates a local variable!!!
    self.code="z" #(5) OK! This is the only case where explicit 'self' is ok
    weight        #(6) OK! You can call a private method defined in a base class
  end
end

• Ruby's behaviour on lines (1), (2) and (5) seems reasonable.
• The fact that (6) is ok is a bit strange, especially coming from Java and C++. Any good reason for this?
• I really do not understand why (3) fails ! An explanation, anyone?
• The problem on line (4) looks like an ambiguity in the grammar, which has nothing to do with 'private'.

Any ideas?