Because with a good manager that isn't either a charlatan like AVB or a complete Ihni binni dimi diniwiny anitaime like Tim Sherwood Paulinho is a much better player than Jordan Henderson. He is way beyond his level.
A bit like how United don't have the 7th best side in the league we just have a terrible manager, when we had a real manager last season we won the damn thing with virtually the exact same team. Or are all our player mediocre now when they were great just four months before Moyes started the season?
Squads, systems, tactics all that stuff is put together by the manager. They effect how well the player performs in the context of his current side but they don't affect a players individual quality outside of his current team (not in the short term sense).
Riquleme for example when he went to Barcelona was played on the wing and generally was rubbish, then he went to Villarreal and was immediately one of the best players in the world. He didn't just suddenly become way better at football because he moved 30 miles down the Spanish coast. He had a manager who got the best out of his talents.
With a good manager I think the Arsenal, Chelsea and Spurs midfield threes all have a higher ceiling than the Liverpool midfield three's but right now only the Chelsea midfield is actually being used in a way where they can show it (although imo if they were fit Ozil, Ramsey, Flamini) would play better than any Liverpool midfield three too, they're just always injured between them.