ruby not preserving order of array elements when sorting

gistfile1.rb

1.9.3p0 :001 > Z=Struct.new(:n,:v)
 => Z 

1.9.3p0 :002 > z=[ Z.new('b',1), Z.new('b',2), Z.new('a',1)]
 => [#<struct Z n="b", v=1>, #<struct Z n="b", v=2>, #<struct Z n="a", v=1>] 

1.9.3p0 :003 > z.sort_by{ |z| z.n }
 => [#<struct Z n="a", v=1>, #<struct Z n="b", v=2>, #<struct Z n="b", v=1>]

1.9.3p0 :004 > z.each_with_index.sort_by{ |z, i| [z.n, i] }.map{|z,_| z}
 => [#<struct Z n="a", v=1>, #<struct Z n="b", v=1>, #<struct Z n="b", v=2>]