Differences between ancestors of singleton class between Ruby v2.0 and v2.1

singleton_class_ancestors.rb

# ruby 2.0.0p0 (2013-02-24 revision 39474) [x86_64-darwin12.2.0]
class X; end
X.ancestors # => [X, Object, Kernel, BasicObject]
X.singleton_class # => #<Class:X>
X.singleton_class.ancestors # => [Class, Module, Object, Kernel, BasicObject]

# ruby 2.1.0p0 (2013-12-25 revision 44422) [x86_64-darwin13.0]
class X; end
X.ancestors # => [X, Object, Kernel, BasicObject]
X.singleton_class # => #<Class:X>
X.singleton_class.ancestors # => [#<Class:X>, #<Class:Object>, #<Class:BasicObject>, Class, Module, Object, Kernel, BasicObject]

# The following are extra ancestors in Ruby v2.1
# [#<Class:X>, #<Class:Object>, #<Class:BasicObject>]