Last year, Pirates' pitching, as measured by ERA, was worst in Major League Baseball. So far this year, it is ranked first. How did they go from "worst to first" so quickly? Things aren't that simple.
The first thing to note is that the Pirates pitchers' "FIPS," or what I call sabermetric ERAs, are quite a bit higher than their actual ones. That is to say, they should have given up more hits and runs, based on the number of their strikeouts, free bases (walks and hit by pitches), and home runs yielded, than they actually did.
Anything of this sort can happen in the short term. Longer term, the Pirates need to improve their raw statistics to maintain their pitching leadership.
Based on the FIPs, the Pirates have a top of rotation hurler in Paul Maholm, two "third starters" in Zach Duke and Ross Ohlendorf, and two "fourth starters" in Ian Snell and Jeff Karstens, basically a league average rotation. That's not like last year's "Keystone Cops," but these guys also aren't yet the Spencer Tracys of baseball.
And there's more. Despite "league average" raw talent, the Pirates have ten quality starts out of fifteen, the highest in the majors. Nine of them are HIGH quality starts (two or fewer runs, not three, in six or more innings), including four shut outs. Most of the remaining games were either awful, or could have been. So even if their aggregate statistics aren't so hot, the Pirates pitch enough good games to win a lot.
In their first fifteen games, the Pirates scored 70 runs and gave up 50. The sabermetric Pythagorean formula using runs scored versus runs allowed suggests that they should be 10-5. In fact, they are only 9-6. On FIPs, however, the Pirates should have allowed about 60 runs instead of 50. Plugging that number into the Pythagorean formula gives a theoretical win percentage that rounds to 9-6.
This points to a last fact. Many of the Pirates' victories have been by large margins (e.g. 10-0, 10-2). In other words, they gave up "too few" runs in games that they were going to win anyway. This gives them something of a cushion in case they start giving up the statistically indicated number of runs. Such runs might not hurt them, if they merely lower the margin of victory, rather than swing games.