Genuine rarity simply shows that you obtained the item the way it was meant to be obtained. This only applies to certain items. For example, it is impossible (to my knowledge) to obtain a genuine, say, Boston Basher, but you can get a genuine Three Rune Blade. It shows, in the case of the blade, that you actually pre-ordered Witcher 2 and got it that way, and didn't just buy it from Mann Co or craft it. They're also rare, so it's worth more in trades (like Powerlord said). In the case of the Bison, it would show that you actually went to the event (or got your hands on a code) instead of buying or crafting it. make sense?