I don't understand the behaviour of unichr():

Python 2.7a0 (trunk:68963M, Jan 30 2009, 00:49:28)
>>> import unicodedata
>>> unicodedata.category(u"\U00010000")
'Lo'
>>> unicodedata.category(u"\U00011000")
'Cn'
>>> unicodedata.category(unichr(0x10000))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: unichr() arg not in range(0x10000) (narrow Python build)

Why unichr() fails whereas \Uxxxxxxxx works?

>>> len(u"\U00010000")
2
>>> ord(u"\U00010000")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: ord() expected a character, but string of length 2 found