QCollator::compare returns values outside its documented range

In the documentation for QCollator it states that it:

Returns -1, 0 or 1 depending on whether s1 is smaller, equal or larger than s2.

This is not the behaviour I'm seeing on Linux - for example comparing "Sambia" and "Samoa" (in the de_DE locale) returns -13. As far as I can see it is simply calling std::wcscoll which is definitely not (always) returning either -1, 0 or 1.

I'm not sure whether this is a documentation error or implementation bug though.