Can someone help me to figure this out...

Why is it that there's 2 pieces that make a set through Sygnery but there's only 1x Ra/Ex version despite the fact that it's says set:- XXX %

For example

Head



Body



But the Ra/ex has

Body



but no head piece, Could Camate or anyone please clarify this? as why would you even bother using the term "set" if you aren't gonna have 2 pieces that make a set bonus from the Ra/Ex version?