The Pirates are succeeding because of their pitching (2nd in ERA, 19th in Runs scored). They're not particularly great at allowing walks (4th most) or striking out batters (11th most). It all come down to the fact that they don't allow many hits. They're #1 in lowest total batting average allowed, bases empty, runners on and runners in scoring position.
They're 3rd in lowest slugging allowed with the bases empty because they allow a fair number of home runs (7th lowest), but it's with runners in scoring position that they become truly extraordinary. The next closest in lowest slugging allowed with RISP is Boston at .348, which is an astounding 41 points higher (if you go 41 points higher than Boston, you'll get to Miami at #13). The Pirates have allowed the 14th most walks, but the fewest hits and fewest home runs.
Will it catch up with them at some point? Maybe, but we're already halfway through the season. It seems to be working for them. The might not have a lot of clutch hitting (#26 in hitting with RISP), but they seem to have plenty of clutch pitching.
This is intriguing, although it might end up not being much at the end of the season. It is very difficult to finish the season with such a low ERA as a reliever because one or two bad outings can wipe out many good outings. I went back and looked at how many pitchers were at this level (20 IP, 2.25 ERA) at the All-Star Break the last few years.
It's similar to the numbers with the explosion of strikeouts in the past few years. After hovering in the 20's and low 30's for a decade, it has moved up dramatically since 2010. There are also 9 pitchers right now with between 15 and 20 innings pitched and an ERA under 2.25. It wouldn't be surprising if the number at the All Star Break this year is in the mid-50's.
Then the question will be how many of them can hold onto it for the rest of the year. If there are 40 or more pitchers at the end of 2013 with at least 40 IP and an ERA under 2.25, that would show how much the game has changed.
The increase in dominant relievers who can strike batters out almost at will without allowing many hits or walks has tipped the balance away from the hitters in a way that might be difficult to reverse any time soon.