Why is this?

[gene tani]

I think you got your answer.  FWIW, questions like this i usually look
1st in the Cookbook (i.e. the Oreilly dead trees, 2nd edition).  See
Chap. 4  And: My gotcha list,

http://www.ferg.org/projects/python_gotchas.html
http://zephyrfalcon.org/labs/python_pitfalls.html
http://zephyrfalcon.org/labs/beginners_mistakes.html

http://www.onlamp.com/pub/a/python/2004/02/05/learn_python.html
http://www.norvig.com/python-iaq.html
http://www.faqts.com/knowledge_base/index.phtml/fid/245
http://amk.ca/python/writing/warts