I have an academic interest in drug trafficking patterns, but I don't know very much about the US situation; however isn't much of the US supply smuggled in over land from Mexico? That would make it quite logical for areas near borders to have high quality supply.
I haven't smoked in years because it doesn't agree with me anymore, but when I did, the stuff from Mexico tended to be lower quality. The good stuff was usually local and grown with care. Think industrial versus local produce.
But then again, the people smoking shwag probably aren't reading hacker news and adding their cost of weed to an online database.