Need new tires

The wheels are cast aluminum with only a 6 lug pattern. The tires are ST 225/70R-15 load range D. The lines I posted above are cast into the back side of the spokes. I have continued looking and have found a chart that references the lower of the 2 tire max weights as load range C @ 50 psi. I have also seen references to the upper weight as load range E @ 80 psi. Is there a reason why they put both on the wheel? Is it a from/to rating? I did go to the local Discount Tire and all that kid could do is punch numbers into a computer. The choices around here for tires are Discount Tire, Wal-Mart and Firestone.