[selectors4] Why is the :only-child pseudo-class not a special case in terms of specificity?

In both the Selectors recommendation and the Selectors4 working draft, the
:only-child pseudo-class documentation states:



> “Same as ‘:first-child:last-child’ or ‘:nth-child(1):nth-last-child(1)’,
but with a lower specificity.”



As much as it would go against the basic specificity implementation,
wouldn’t it be logical for the :only-child selector to have the same
specificity as those combined selectors?



- James