Why do Tufenkian's handmade carpets vary in size?

Tufenkian carpets differ from machine-made carpets in several ways, including variations in size. These variations exist because of the numerous handmade processes used to make each rug. The variation in size ranges from +/- 2% in width and +/- 4% in length. Our wool is spun into yarn by hand; therefore, the yarn varies in diameter, which results in knots of varying size. The thinner the yarn, the smaller the knot; the thicker the yarn, the larger the knot. So it follows that the smaller the knots, the smaller the rug; and the larger the knots, the larger the rug. Additionally, during the weaving process, each row of knots is beaten down by hand. This also contributes to variations in size. Nominal sizes in the Tibetan Collections include 6'x9' (actual approximates 5'6"x8'6"), 9'x12' (actual approximates 8'9"x11'6") and in the Armenian Collections the 5'x7' (actual approximates 4'6"x6'6").