The format used by Microsoft Office differs from the ECMA spec (how much I'm not sure). By default it saves it in a different format. You have to choose a Strict filetype for that format to be used. Meaning, most files out there don't follow the spec. Further, some bits of the spec are quite strange ("follow the behaviour of Word/WordPerfect"). See also https://en.wikipedia.org/wiki/Office_Open_XML.