I wasn't planning a second part to this story, but Tom Gower of Total Titans asked if the division effect might not be come from the second matchup between two teams. The theory being that "the argument may be more specific familiarity rather than general familiarity". If that were the case, we'd see closer games in rematches each year.
It seemed like a plausible explanation, so I tested it:
First matchup: Colts 7-2. Average Score: 22.0-16.1
Second matchup: Colts 6-3 Average Score: 26.4-26
Closer final score: Game one 6, Game two 3
Final score within five points: Game one 4 Game two 4
The Jags fared a better against Indy the second time around, but in a sample size of 9 games, one massive outlier skews the deck. The 2006 thumping (Jags 44 Colts 17) really alters the final averages making the 'second game' totals much closer than they were in a typical year. In terms of score, however, the first game of the matchup ended with a closer final score more often than the second game.
First matchup: Colts 7-2. Average Score: 25.3-17.6
Second matchup: Colts 6-3 Average Score: 25.8-17.1
Closer final score: Game one 7, Game two 2
Final score within five points: Game one 3 Game two 3
Again, the first games of the matchup was closer than the second games. The Titans won an extra game in the second matchup (perhaps the 16-10 win at Indy in 2007 when Manning sat), but their average points were worse and the first game was often the closest of the matchup.
First matchup: Colts 8-1. Average Score: 32.0-20.4
Second matchup: Colts 8-1 Average Score: 28.1-18.2
Closer final score: Game one 4, Game two 5
Final score within five points: Game one 2 Game two 2
The margin of victory for the Colts shrank from 11.6 to 9.9 in the second game, but that's the only evidence they were closer. What's really interesting is that in the first five years of the Texans existence, the second game of the series was always the closer one. Once the Texans became truly competitive in 2007, however, that flipped. Since 2007, the FIRST game has been tighter than the second.
There's not a lot of evidence that specific familiarity causes closers games between the Colts and other teams in the AFC South. The first game of the matchup tends to be the closer of the two. Again, the overarching myth that 'division game are tougher' simply doesn't seem to hold water.
Given a second crack at the Colts, the division didn't fair too much better going 5-22 the first time and 7-20 the second time around.
The first time the division played the Colts the average score was 26.4-18.0. The second time the score was 26.8-20.4. Again, a little closer, but not much.
The first time the division played the Colts in a season, the score was within 5 points 9 times, the same as the second. The first game of the year was the closer game 17 times, with the second game being closer 10 times.