Case equality weirdness

ruby-1.9.2-p180 :004 > class A
ruby-1.9.2-p180 :005?>   def ===(other)
ruby-1.9.2-p180 :006?>     puts "A#===(#{other.inspect}) called"
ruby-1.9.2-p180 :007?>     end
ruby-1.9.2-p180 :008?>   end
 => nil
ruby-1.9.2-p180 :009 > class B
ruby-1.9.2-p180 :010?>   def ===(other)
ruby-1.9.2-p180 :011?>     puts "B#===(#{other.inspect}) called"
ruby-1.9.2-p180 :012?>     end
ruby-1.9.2-p180 :013?>   end
 => nil
ruby-1.9.2-p180 :014 > case A.new
ruby-1.9.2-p180 :015?>   when B.new
ruby-1.9.2-p180 :016?>   end
B#===(#<A:0xa2e7930>) called
 => nil