Something is weird about the NFL Schedule?

Ok so I was looking at the 2011 NFL Schedule, and I noticed the Chiefs were playing the Lions in Detroit this season. Now the weird thing is this happened in 2007, and the NFL usually alternates the home team for interconference games, so the fans can see every NFL team at their home team's stadium, but for some reason this game was not alternated. Then I looked at the rest of the Chiefs' schedule, and I noticed that they play all the NFC North teams in the same stadiums as 2007. After some more investigating, I noticed this is the same case forr all teams in the AFC West (Chargers, Raiders, Broncos), as well as the NFC West (Cardinals, 49ers, Rams Seahawks). Why is this?