Two things make ranking pitchers difficult. Both have to do with enforcing a static structure (rankings that publish here and now) upon an ever-changing sport.For one, the pitchers themselves are a moving target. They’re adopting new pitches, throwing the ball harder and playing through injury. They’re also facing tougher and weaker opponents. So under-the-hood changes don’t always come to light right away.Consider the case of Dodgers left-hander Justin Wrobleski. He was 92nd in the last rankings because he had not shown a strikeout rate over 20 percent. Repeated watchings confirmed my biases that he didn’t have what it took to turn foul balls into whiffs and would always be limited by that low strikeout rate despite his elite command. Pitchers with a sub-20 percent strikeout rate have a 1-in-124 shot at being a top-15 pitcher and a 1-in-5 shot at even being a top-75 starter, so the ranking seemed justified.Well, then — as young pitchers do — Wrobleski improved himself. He threw his fastball a tick faster, started throwing the sinker twice as much as he had been, featured the splitter more often and recently debuted a sweeper. His strikeout rate went up. Since that last ranking, he has a 27-percent strikeout rate against that always-excellent walk rate, and now the question is just how much further up the rankings he’ll go.