I think you're on to the right answer (though I don't know for sure myself). It seems to me that if all file format identifiers started at the zero offset, it

I think you're on to the right answer (though I don't know for sure myself). It seems to me that if all file format identifiers started at the zero offset, it would be impossible for a single file to identify as more than one format. However, when different formats use different offsets to identify themselves, it is possible to construct the file in such a way that it validly identifies as more than one format.