Because he passes no more than 10 yards. Ruben is never going to light up pass completion stats when he's asked to play the diagonal like he does.I mean, he completes the same percentage of his passes as Moutinho (and better than Ruben, too!) but hey ho.
He's basically the epitome if true defensive mid for me. Endless engine, safe passer, positionally sound, and can even play a good long ball when required.