Every rug is an individual, like its weaver. Many traditional weavers still depend upon their own resources. They raise the sheep, shear, wash, comb, and spin the wool. They then dye it as taught by their mothers using, in many cases, native plants.
In buying a weaving, consider if it is symmetrical with a nicely balanced overall design. However, the real reason should be if you fall in love with it and the colors are right.