Xml schema defines a usage profile which is a subset of the iso8601 profile. this profile defines that the following is the only possible representation for a datetime, despite all other options iso provides.

yyyy-mm-dd"t"hh:mi:ss(z|[+-]zh:zm)
datetime::format::xsd is a subclass of datetime::format::iso8601, therefore it will be able to parse all other iso options, but will only format it in this exact spec.