One common mistake when counting knots per square inch (KPSI) in an oriental rug is to count each individual Yarn Ply rather than an actual knot as a whole entity.
Yarn is spun into one "staple," which although always is comprised of several dozen or more individual hairs, can also be combined with several other staples to make a multi-ply yarn. When a carpet is then counted for kpsi, it can be easy for the novice to mistake a triple ply yarn as three individual knots. This can throw off KPSI by a significant amount. A carpet which has a two ply yarn and 100 KPSI (10 knots horizontally x 10 knots vertically) could theoretically be mistakenly advertised as 200 KPSI, or 20 knots vertically and 10 knots horizontally.
Always take a careful look at how the yarn is spun in one individual knot. Take a look at how to properly count knots in your rug. If there is ever any doubt, bring it to our rug forum!