Странный результат в python

Может ли кто-нибудь объяснить мне этот странный результат на python 2.6.6?

>>> a = "xx"
>>> b = "xx"
>>> a.__hash__() == b.__hash__()
True
>>> a is b
True # ok.. was just to be sure

>>> a = "x" * 2
>>> b = "x" * 2
>>> a.__hash__() == b.__hash__()
True
>>> a is b
True # yeah.. looks ok so far !

>>> n = 2
>>> a = "x" * n
>>> b = "x" * n
>>> a.__hash__() == b.__hash__()
True # still okay..
>>> a is b
False # hey! What the F... ?