The main purpose of coating wick is to add rigidity to the assembly when it is inserted into a container. A proper wick coating wax is hard but pliable and has a reasonably high melt point. Many different types and blends of wax can be used to coat wick without affecting the performance of the wick.
CandleScience has recently switched all of our wicks to a natural coating. Natural candle makers can market their products as 100% natural wax all the way down to the wick, and paraffin candle makers don't notice the difference.
An interesting thing to note; once a wick is coated with wax, it is nearly impossible to determine the small difference between various wick sizes. Because the type or amount of wax added to the wick does not affect the burn the thickness is not precisely controlled. So while raw LX-10 has a smaller diameter than raw LX-18, they may appear similarly sized once coated.
Often people receive a new bag with a little more or less wax than their previous batch and suspect they received the wrong wick. Don't worry, it's just coating thickness, you can trust the wick is exactly as labeled.