If you add yourself to a list and then call the get list members method you might be surprised by the result.
One of the data items returned by the get list id method includes the number of members in your list. The get list members method returns 20 members at a time and you need to call the it multiple times using different cursor locations to retrieve the complete membership. When you count them and compare against the number returned by the get list id method, you will find that they do not match. Your own account is not part of the dataset that is returned.
By the time you are reading this post, the list might have changed. So I have saved a copy of the XML here. On the other hand when you access the list through twitter.com, you can see that I am included in my own list. In fact my screen name (e4c5) appears twice!
The work around I suppose is to not rely on the members count returned by the get list id method.