The documentation for this test seems to describe exactly what the test is doing:

"any element instance which matches the wildcard is required to have a governing type definition compatible with the type assigned by the local element declarations matched by the element's expanded name."

The instance document (specifically, the second <x> element) appears to violate this condition, so it is unclear why it is tagged as valid.

The confusion may be something to do with the spurious xsi:schemaLocation attribute on the instance document, which refers to a namespace "a" that is not actually used in the test, with a reference to a schema document whose targetNamespace is actually "b".