Python's os.listdir behaviour on Windows

>>> import os
>>> os.chdir('c:/python27')
>>> os.listdir('c:')
['Tools', 'include', 'python.exe', 'libs', 'DLLs', 'Lib', 'NEWS.txt',
'w9xpopen.exe', 'Doc', 'pythonw.exe', 'LICENSE.txt', 'README.txt', 'tcl']
>>> os.listdir('c:/')
['users', 'Program Files', 'Python27', 'windows']

Why is the "/" after "c:" affecting the outcome? Is there a way to get os.listdir('c:') to return the contents of "c:/"?