A sample output (either XML or JSON) is not a substitute for a specification.
Looking at the XML when a list has only a single element, you wouldn't know if there could be more. You need an XSD to accompany the XML, which is what is typically provided.
When looking at a JSON document you might run into the same situation where the document you received doesn't contain all possible elements, and you wouldn't be the wiser. You'd still need a proper JSON schema to accompany the document.
Looking at the XML when a list has only a single element, you wouldn't know if there could be more. You need an XSD to accompany the XML, which is what is typically provided.
When looking at a JSON document you might run into the same situation where the document you received doesn't contain all possible elements, and you wouldn't be the wiser. You'd still need a proper JSON schema to accompany the document.