When Alex Smith was good Sunday, he was very good. When he was bad, well, he was bad enough to finish with only the 24th-best QBR score for Week 4.

The San Francisco 49ers quarterback posted a 99.8 QBR during the third quarter of his team's memorable comeback victory at Philadelphia in Week 4. It was the second-best quarter for a quarterback Sunday and enough, with a huge fumble recovery from the 49ers' defense, for the 49ers to prevail 24-23.

But that one quarter and the 49ers' ultimate victory could not prevent Smith from finishing with a 28.7 QBR overall. That is partly because running backs Kendall Hunter and Frank Gore scored the pivotal touchdowns late in the game, whereas Smith's touchdown passes came when the 49ers were still trailing by wider margins.

QBR measures performance in part by taking into account win probability from play to play. The plays that improve win probability the most impact QBR scores far more than similar plays in less critical situations.

For example, Smith's scoring pass to Josh Morgan with 7:20 left in the third quarter improved the 49ers' win probability from 2.9 percent to 6.3 percent, based on how similar plays in similar situations have affected outcomes previously. The probability improved from 12.2 percent to 19.1 percent with Smith's scoring pass to Vernon Davis later in the quarter.

The touchdowns from Hunter and Gore each improved the 49ers' chances by more than 10 percent, but the 49ers' win probability still stood at only 51.1 percent after Gore's touchdown.

"This gave the 49ers the lead, but still left three minutes for the Eagles, who needed only a field goal to win," Albert Larcada of the ESPN analytics team explained. "Jeremy Maclin’s fumble sent the 49ers' win probability from 39.7 percent to 74.9 percent, by far the biggest swing in the game. Obviously, Alex Smith had nothing to do with that swing."

After the Maclin fumble, Gore ran the ball five times as the 49ers put away the game. Smith completed 4 of 8 passes for 22 yards and no first downs with a sack in the fourth quarter, costing his team about one expected point.

"So, essentially, Smith started the 49ers' comeback with a great third quarter, but that was only enough to get the 49ers to roughly a 20 percent win probability," Larcada said. "From there, Smith had very little to do with the 20 percent becoming 100 percent."

Agree or disagree? I'll be disappointed if no one writes a passionate response claiming they do not care. My take: Smith played better when the score was lopsided than when it was close, and QBR sniffed this out, as intended. But Smith's QBR takes nothing away from a memorable victory. He had his moments during a highly satisfying team victory, and he should enjoy it.

Quick thoughts on why NFC West passers graded out as they did by Total QBR in Week 4, with NFL passer ratings in parenthesis as a reference point:

Tarvaris Jackson, Seattle Seahawks (68.2 QBR, 96.3 NFL rating): Jackson completed 25 of 38 passes for 319 yards with three touchdowns and two interceptions. Jackson took no sacks and that was a critical factor in helping Jackson score easily his highest QBR of the season. The Falcons didn't get pressure. The Seahawks responded by spreading the field and trusting Jackson to a degree they had not trusted him, in part out of necessity after falling behind. This was a tremendous performance from Jackson. He made good use of an improved group of targets, including rookie Doug Baldwin.

Kevin Kolb, Arizona Cardinals (32.0 QBR, 67.9 NFL rating): Kolb completed 20 of 34 passes 237 yards with no touchdowns and one interception. Kolb took four sacks and lost a fumble. Sacks continue to work against him in the QBR rankings. The Cardinals also struggled on third down, converting only three times in 12 chances. Unlike recent performances, Kolb didn't have touchdown plays on his side to mitigate the negatives.

Alex Smith, 49ers (28.7 QBR, 112.1 NFL rating): Smith completed 21 of 33 passes for 291 yards with two touchdowns and no interceptions. He took three sacks and fumbled twice, losing one. This was the most memorable and significant victory of Smith's career by just about any measure. He completed all nine attempts in the third quarter and played a big role in getting the 49ers going, even if he didn't make the critical plays to win. QBR said this performance was better than only 28.7 percent of quarterback performances since 2008. That's a tough sell even if the math holds up.

Sam Bradford, St. Louis Rams (12.8 QBR, 64.5 NFL rating): Bradford completed 20 of 43 passes for 164 yards with one touchdown and no interceptions. He took seven sacks, averaged 3.8 yards per attempt, lost a fumble and never scrambled one week after suffering a toe injury against Baltimore. Bradford appears overwhelmed at the moment. The Rams' new offense asks him to give receivers additional time to get open, but with protection issues persisting and receivers too often appearing covered, Bradford appears to have few viable options. And when he does deliver throws accurately, receivers are dropping them too frequently. At this point, the Rams are risking longer-term damage to their quarterback. Something must change.

Teams with higher QBRs than their opponents went 11-4 (.733) in Week 4. Carolina, Philadelphia, Dallas and Buffalo lost despite higher QBRs.

Teams with higher NFL passer rating were 10-5 in Week 4. The Panthers, Cowboys, Bills, Atlanta Falcons and Washington Redskins lost despite higher passer ratings Sunday.

Teams with higher QBRs are 51-12 (.810) this season. Teams with higher NFL passer ratings are 45-18 (.714).

2011 Relevant NFC West QBR Ranks: Week 4