He is improving them and the rest of his physique all the time. He's not like Shawn Ray or Dex, who look(ed) the same year after year.
Compare pics of him from last year or when he turned pro to this year's Mr Olympia and you'll see the difference.
If he doesn't get injured and continues to improve, he'll look even more cartoon-ish.
When you have few flaws, like Jackson or Ray, your focus becomes conditioning.
Plus saying that Jackson looked the same, year after year, is ridiculous. He has slowly added mass to his frame, while keeping those signature cuts that earned him the nickname, "The Blade".
And, because he's almost always in top shape, he caught Jay Cutler napping last year, much as Ray did with Yates in 1994. But, unlike Yates in 1994, the judges didn't let the defending water-logged champion keep his title that year.
That's why Jay Cutler had to come SLICED, to get his title back.