At the end of yesterday's mammoth Zach Duke post, Azibuck chided me for not explaining my conclusion that much of Duke's improvement this year has been due to the Pirates' improved defense. This year, the Pirates are turning 73.1% of balls in play against him into outs, which is way up from 68.5% last year and and 63.1% the year before. His BABIP has dropped considerably, and the difference between his ERA (3.29) and his FIP (4.18) is fourth in the National League among guys with more than 100 innings. Defense is playing a big role in Duke's success this year.
So why, then, is the defense not helping Paul Maholm? Maholm's strikeouts are down this year, but his K rate is still higher than Duke's. Their line drive percentages are almost identical, Maholm gets more ground balls, and he's kept 11% of flyballs in the infield this year, much better than his career rates and much better than Duke's IF/F %. Maybe because of that, his home run rate is way down this year as well. The biggest difference for Maholm is that the DER behind him has dropped from .710 last year to .674 this year.
Being upfront here, I don't really have an answer to the question I posed at the header here. One possible explanation is that Maholm has made 11 starts on the road compared to just seven at home while Duke's got an even 9/9 split. PNC has graded out as a pretty solid pitcher's park the past few seasons and the big left field makes it especially friendly to lefties. There, Maholm's picked up three of his wins with a 2.96 ERA and a 1.25 WHIP. On the road, he's 3-3 with a 5.77 ERA and a 1.63 WHIP. Duke also has a noticeable home/away split, though it's considerably less drastic.
Is it luck? I hate (hate hate hate) labeling something as luck, but I'm at a loss to explain how two pitchers with such similar approaches with such similar batted ball data have such varying defensive results. There are a few metrics, notably David Pinto's PMR, that quantifies defense behind pitchers, but those generally aren't released until the end of the season and until I see them, it's going to be hard to quantify exactly what's going on with Maholm.
As for his second half, there are two things that could happen for him. One is that things start evening out, he puts some good performances together, and his ERA comes down a bit. He's the most extreme groundball pitcher on the club, so the loss of Nyjer Morgan's defense in left theoretically affects him the least. The second is that his HR/F rate (the percentage of flyballs he gives up that become home runs), which is an uncharacteristically low 5.5% (he's usually between 12-14%) begins to catch up with him and while he gets better defensive support, his home runs spike and things don't change superficially.