On July 21, Jered Weaver recorded his 13th win of the season. His box score recorded his seven innings, two walks, and six strikeouts to go with his zero runs and seven hits. His box score even recorded his 122 pitches—the sixth time this year he has thrown 120 pitches or more. Next to his box score, Weaver sits atop the leaderboard for ERA. But Weaver’s box scores do not tell the whole story.

Game stories might shed more light on Weaver’s performance. They might talk about how C.J. Wilson, who pitched eight innings and allowed just one unearned run while striking out eight, was a hard-luck loser. They might talk about how good the Angels’ defense was behind Weaver. They will almost certainly describe the seven hits Weaver allowed as “scattered.” The AP’s version of the story even included the following description for flavor: “The righthander’s biggest out by far was in the sixth, when Weaver struck out Mitch Moreland on an elevated 2-and-2 fastball with the bases loaded. He punched the air after walking off the mound.”

But even that does not explain the entire mystery of Weaver’s fantastic 2011. After yet another superb start (a 7.0-1-1-2-5 piece of business-as-usual that gave him his 14th win of the year) on Tuesday night, Weaver had a 1.79 ERA. As you might expect for the league ERA leader, his FIP was higher: 2.40. But this is not Weaver’s first time at the rodeo. In his six seasons in the majors, Weaver’s ERA has been below his FIP and FRA five times. His career ERA is 3.29, while his career FIP is 3.58.

Before this season started, the conventional wisdom held that Weaver’s 2010 marked his arrival on the front lines. Where he had once been a 1-A starter—an ace only by default and certainly not with someone like Dan Haren ahead of him—last year he became a true ace. He started 34 games, threw over 200 innings, and (most importantly) struck out more than a batter per inning. Even WARP reflected Weaver’s improvement in 2010: after slowly climbing from 3.1 to 3.4 to 3.6, he jumped to 5.1 last year. His peripherals improved across the board, showing lower walk and home run rates to boot.

Here is the heart of the Weaver mystery. In 2011, his strikeouts are back to 2008–09 levels, yet he is allowing runs at an extraordinarily low rate. He has allowed one or zero runs in an amazing 14 of his 22 starts. Undoubtedly, much of this improvement is attributable to the decline in Weaver’s home runs. Through 161 innings, Weaver has allowed just six home runs despite his traditional fly ball tendencies. Some of that is due to the fact that Weaver has faced weak opponents: batters he has faced have posted an OPS that ranks Weaver 117th out of 130 (among pitchers with at least 80 IP) in that category. But not all of the blame can be placed on the shoulders of the Chone Figginses and the Cliff Penningtons of the world.

The ace up Weaver’s sleeve has long been his ability to induce batter pop-ups. In fact, since 2006, Weaver has the best pop-up rate of any pitcher with at least 700 IP. Here is a top 10 list (rate in parentheses):

1.      Jered Weaver (13.8%)
2.      Ted Lilly (13.4%)
3.      Scott Baker (13.0%)
4.      Barry Zito (12.5%)
5.      Tim Wakefield (11.7%)
6.      Johan Santana (11.4%)
7.      Matt Cain (11.0%)
8.      Jarrod Washburn (10.9%)
9.      Scott Kazmir (10.8%)
10.  Matt Garza (10.7%)

Weaver’s mark puts him way out in front. Even if we lower the threshold to 300 IP, Weaver still holds his own very well against relievers and other starters who are boosted by a smaller sample:

1.      Chris Young (17.2%)
2.      Robinson Tejeda (15.0%)
3.      Carlos Marmol(15.0%)
4.      Jason Bergmann (14.1%)
5.      Jon Rauch (13.9%)
6.      Jered Weaver (13.8%)
7.      Rich Hill (13.7%)
8.      Kevin Slowey (13.6%)
9.      Brian Fuentes (13.6%)
10.  Bob Howry (13.5%)

But this year, Weaver is outdoing himself in the popup department. His mark is at 17.2%. Second place (minimum 80 IP) goes to Jeremy Hellickson at 13.4%. His 76 pop-ups mean he is averaging just over four per nine innings pitched; the major league average is about two. As a result, Weaver gets the benefit of an additional pair of almost-guaranteed outs nearly every start. That has always been enough to keep his BABIP low. While his 2011 mark is an artificially low .249, it is not far out of line with his career mark of .278.

What Weaver is doing is not unprecedented. Sid Fernandez had an ERA well below his FIP (3.36 versus 3.52), a very low BABIP (.247 career), and an excellent track record at inducing pop-ups. Starting in 1989 (the first year for which we have good data on pop-ups), Fernandez had the following ranks in pop-up rate among ERA title qualifiers:

1989 — 1
1990 — 1
1991 — n/a (did not qualify for ERA title)
1992 — 1
1993 — n/a (did not qualify for ERA title, but would have ranked #1)
1994 — n/a (did not qualify for ERA title, but would have ranked #1)
1995 — n/a (did not qualify for ERA title, but would have ranked #1)

Sid the Kid was not generally recognized as an excellent pitcher. He was only twice an All-Star and never was a top-five finisher for the Cy Young. Some of his numbers, especially his home run numbers, were helped by playing so many seasons in Shea Stadium. Jered Weaver very much looks like he will end up having a better career than Fernandez did, but at least this path has been walked before, and Angels fans can rest easy knowing that Weaver could keep up the pop-up rates for quite some time to come.

Thanks to Dan Turkenkopf for research assistance.