.remove() method in for-loop

[nzcan]

it is clear what this code do:

>>> sw = ['go', 'bo']
>>> for cm in sw:
... 	if cm == 'bo':
... 		sw.remove(cm)
... 
>>> sw 
['go'] 

but when a list contains 4 'bo' items only 2 will be removed:

>>> sw = ['go', 'bo', 'bo', 'bo', 'bo']
>>> for cm in sw:
... 	if cm == 'bo':
... 		sw.remove(cm)
... 
>>> sw 
['go', 'bo', 'bo']
>>>  

why not all 'bo' items are removed?