That's typical of most chain parks. Six Flags is the same way. If you buy your pass at SFGAm you'll spend more than at SFKK. It has to do with the size of the park, number of rides, etc. The idea is that most of the passes are bought by locals who don't travel all over the country. So, why should someone who's home park is SFKK pay the same as someone who's home is SFMM? If Vallyfair is your home you don't pay as much as if you live near Cedar Point.
It benefits you if you can get your pass at a smaller park, pay less money, and then use it everywhere else.
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Only two things are infinite, the universe and human stupidity, and I'm not sure about the former." - Albert Einstein