[F&O] fn:subtract-dateTimes-yielding-yearMonthDuration rounding

The description of the fn:subtract-dateTimes-yielding-yearMonthDuration says
in one place that it "returns the result rounded to contain only years and
months."  But it also says that "If there is a remaining number of days,
they are discarded." 

These two statements seem to contradict each other.  Does it round or not?

Thanks,
Priscilla