That's exactly the expected behavior, if you read through the docs for the GetListItems operation. If you want to specify the fields to return (almost always a good idea, as the default view can change), specify them in the CAMLViewFields.
BTW, you don't need to use this:
as SPServices will use the current site or the default value you have set.
You can also use the list name ("Announcements") rather than the GUID, for clarity. It also makes moving between sites or environments painless.