A salt question

I've wondered about this every winter of my life when I've read about cities/counties running out of road salt in Wisconsin: Why? Everybody in this state knows that we have a lot of variance in our winter weather. Some winters are very mild and communities do not have to exhaust their salt supplies. Other winters are more severe, and the need for salt exceeds supplies/budgets. Why aren't communities figuring out an average salt supply and sticking with that year after year? Excesses from mild winters can then carry over until more severe winters. Are our communities really that poor at road salt management that they squander their entire yearly supply, regardless of need?