A. an error in the software that computed the value for the web page, B. a transcription error in putting the checksum on the web page, C. accidental corruption of the file, or D. a deliberate attempt to substitute a file with a similar MD5 sum.
It seems that the answer is probably B. But, according to the release website manager, a mystery remains :
http://mail.python.org/pipermail/python-3000/2008-July/014281.html -- http://mail.python.org/mailman/listinfo/python-list
