ruby assignment method visibility

gistfile1.rb

#!/usr/bin/env ruby

class A
  def a=(x)
    puts "-- a=(#{x})"
    @a = x
  end

  def a    puts "-- a"    @a
  end

  def b    puts "-- b"
    a # executes A#a
  end

  def b=(x)
    puts "-- b=(#{x})"    
    a=(x) # Why does this NOT execute A#a=
  end
end

a = A.new

puts a.a # nil
a.a = 321
puts a.a # 321
puts a.b # 321
a.b = 123
puts a.a # 321???
puts a.b # 321?