The dealership we purchased my wifes new HHR from provided a website to track maintenance and mileage on her car, while looking at the maintenance schedule I saw something I have a question about and hopefully someone here knows the answer. The maintenance schedule is broken into 3k and 6k intervals, but the intervals are set-up somewhat irregular. The recommended maintenance to be performed shows the same items to be checked at every visit, so why can they go 6k sometimes and only 3k at other times?

Recommended Services For My 2008 Chevrolet HHR
Date* Service
3/12/2008 6,000 Mile Service
8/5/2008 12,000 Mile Service

10/17/2008 15,000 Mile Service
12/30/2008 18,000 Mile Service

5/25/2009 24,000 Mile Service
10/18/2009 30,000 Mile Service
3/14/2010 36,000 Mile Service

5/26/2010 39,000 Mile Service
8/7/2010 42,000 Mile Service
10/19/2010 45,000 Mile Service
12/31/2010 48,000 Mile Service
3/15/2011 51,000 Mile Service
5/27/2011 54,000 Mile Service

10/20/2011 60,000 Mile Service
3/14/2012 66,000 Mile Service

5/27/2012 69,000 Mile Service
8/8/2012 72,000 Mile Service
10/20/2012 75,000 Mile Service
1/1/2013 78,000 Mile Service