I suppose the easiest(traditional) answer is that on a single edged knife, the sheath is shaped to hold the knife is such a way that the edge is facing towards the back of the person when they carry it. A right handed sheath would hold the blade so that if the sheath is on the right side of the body and drawn with the right hand, the blade comes out of the sheath that way(back of the knife in front and edge behind). Even if you wear the sheath on your left side for a crossdraw grip, the blade still comes out of the sheath right handed, i.e., the knife is still drawn edge down. Same for left hand. In some cases, particulary in the case of double edged weapons like daggers; if carried on the belt, the sheath is essentially ambidexterous since the blade has two sharp edges and it doesn't matter what side you carry the sheath on. There are also ambitdexterous sheaths for single edged knives out there that allow you to set them up for the "handness" you use, or are designed so the knife can fit into the sheath either way(retangular sheaths. I"m left handed, but make right handed sheaths for my knives. In other words I draw the knife out with the edge forward and the back facing to the rear. After so many years of doing that(try and find a left handed/ambi sheath in the old days), I thought I could finally have a proper left handed sheath once I started making my own knives /sheaths. Nope. Turned out I'm so used to that "edge forward" carry that a proper left handed sheath give me a "clumsy" draw. It' s what you get used to I guess, that matters.
