49ers DVOA by quarter: Anything to make of up and down performances?

Earlier today, in the post about Will Tukuafu and Demarcus Dobbs changing their numbers, 16to80fan made a point about the 49ers points per game by quarter. According to the folks at TeamRankings.com, the 49ers are 20th in points per game in the first half, 9th in the third quarter and sixth in the fourth quarter.

I was curious about the 49ers overall performance by quarter and half, so I decided to swing by Football Outsiders' premium stats. FO rates out teams using Defense-adjusted Value Over Average (DVOA). For those who are not aware of DVOA, the basic idea is that it breaks down every single play and compares a team's performance to a league baseline based on situation. FO is then able to use this to figure out how teams performance in specific situations and timeframes in the game. While DVOA isn't perfect, it does a solid job trying to give us more context than the basic counting stats can do.

I took a quick look at the 49ers offensive and defensive DVOA numbers quarter and half. Here is a breakdown of their ranking in the various situations:

Offense
1Q: 3rd
2Q: 10th
3Q: 3rd
4Q: 20th
1H: 4th
2H: 7th
Overall: 5th

Defense
1Q: 5th
2Q: 25th
3Q: 4th
4Q: 4th
1H: 11th
2H: 4th
Overall: 4th

One thing that is interesting to note for the 49ers offense is that even though they have the 20th-ranked DVOA in the fourth quarter, they rank No. 3 in late and close situations, which includes all plays in the second half or overtime with the score within 8 points.

The 20th ranking on for fourth quarter offense and 25th ranking for second quarter defense certainly stick out. Any thoughts on why those numbers are there? It strikes me as maybe opposing offenses adjusting after the first drive or so, with the 49ers defense then bouncing back with second half adjustments. Any other thoughts these rankings?