I don't really have an explanation because I'm not looking for a way to explain the obvious. When a player who derives a ton of his value from scoring goals isn't scoring goals, he's not contributing to the team as much as he should. That's all. Well, I suppose, additionally I'd say that "playing well" is related to what a player is actually contributing and so I see the "Sure, he's not contributing anything but he's playing just as well as when he is" as a bit of a contradiction.
I don't deny good players go through slumps. Albert Pujols can hit .200 for a month and Kobe Bryant can shoot 5-20, the difference is I wouldn't say that Albert Pujols or Kobe Bryant played as well as they always do during these slumps, they're just missing the ball or the net by the merest of inches and chalk it up to luck or the breaks. Hitting the ball, making shots, scoring goals...missing by inches is the same as missing by feet. They miss. Players playing well don't miss as much. That's how we separate good players from bad.
That's it. Kessel isn't playing to the standard he needs to. It would be the least controversial statement in the world if there weren't people still fighting Brian Burke's battles.