PC, please understand that I'm no expert, but it's my understanding that they are khukuris made from "common" steel, such as that from which railroad tracks are made. Whereas the better khuks are made from junked automobile leaf springs, which are made from top-grade steel. It is also my understanding that while rail common khuks are, by and large, inferior, they are not necessarily bad and can be quite functional -- just not as good as the "leaf-spring" khuks. I guess, as with most things in this ol' world, it all boils down to the old saying: "You get what you pay for."
Perhaps some other forumites will contribute their perspectives regarding your question -- we have some very knowledgable folks who make up the forum.