Why do ruby setters need “self.” qualification within the class?

#!/usr/bin/ruby

class A
  def qwerty; @q; end                   # manual getter
  def qwerty=(value); @q = value; end   # manual setter, but attr_accessor is same
  def asdf; self.qwerty = 4; end        # "self." is necessary in ruby?
  def xxx; asdf; end                    # we can invoke nonsetters w/o "self."
  def dump; puts "qwerty = #{qwerty}"; end
end

a = A.new
a.xxx
a.dump

using System;

public class A {
  public A() {}
  int q;
  public int qwerty {
    get { return q; }
    set { q = value; }
  }
  public void asdf() { qwerty = 4; } // C# setters work w/o "this."
  public void xxx()  { asdf(); }     // are just like other methods
  public void dump() { Console.WriteLine("qwerty = {0}", qwerty); }
}

public class Test {
  public static void Main() {
    A a = new A();
    a.xxx();
    a.dump();
  }
}

using System;

public class A {
  public A() {}
  public int test { get { return 4; }}
  public int useVariable() {
    int test = 5;
    return test;
  }
  public int useMethod() {
    int test = 5;
    return this.test;
  }
}

public class Test {
  public static void Main() {
    A a = new A();
    Console.WriteLine("{0}", a.useVariable()); // prints 5
    Console.WriteLine("{0}", a.useMethod());   // prints 4
  }
}

qwerty = 4

class A
  def test
    4
  end
  def use_variable
    test = 5
    test
  end
  def use_method
    test = 5
    self.test
  end
end
a = A.new
a.use_variable # returns 5
a.use_method   # returns 4

var = expr    // assignment
  method = expr // setter method invocation

var = expr    // assignment (new or existing)
  // method = expr, disallow setter method invocation without .

symbol = expr // push 'symbol=' onto parse tree and decide later
                // if local variable is def'd somewhere in scope: assignment
                // else if a setter is def'd in scope: invocation