Why are some used games cheaper than other used games?

Specifically, why are two of the same used games usually different prices?

Last week I bought Halo 3 for $11.99. It was sitting next to two other copies of Halo 3, both selling for $17.99, one of which was in worse condition than the $11.99. I’ve played through most of the content with no trouble at all, and there’s no scratches on the disc. It had the manual, the the ads they throw in the box, everything basically.