This is something most people know, that the Medium Armor Rare drop isn’t what should be. It’s named Pirate (like the green and blue drop) and have the Rogue skin (a low-mid level areas cheap Karma Set). The only piece that matchs the real armor set is the mask (while still named Pirate) which indicates that the full set is supposed to be the one attached at the end of this post.
The only way to obtain it (and I think you can’t even complete the set) is through Personal Story.
I think this should’ve been fixed looooooong time ago, Heavy and Light armors have their Rare set correct and it’s good to see armors of more visual quality the rarer they are. Having this set with the same skin of a very cheap karma armor instead of the one it should be (which IMO is cooler) feels wrong.
It’s about time this get fixed and I’m sure it’s as easy as changing some words in the code for Rare Drop Medium Armor so it has the correct skin, the skin is in the game so it seems to be a problem of the drop finding a path to a different skin when “Rare Tier” condition is met.
There are other Medium sets missing, particularly the Karma equivalent to Heavy Plate and Stately (supposedly this one: http://argos-soft.net/GW2ArmorGallery/index.php?weight=2&race=2&color=10&sex=0&armorSet=highwaymani) and the Headpiece of the Karma equivalent to Dark Armor and Tactical armor (though that one is available in WvW at least, but it should be available with the rest of the karma set: http://argos-soft.net/GW2ArmorGallery/index.php?weight=2&race=2&color=10&sex=0&armorSet=markyi).
This really looks like the variables corresponding to the Medium Armor category got mixed at some point in development and now some of them point to the wrong skins or simply aren’t there. Knowing which ones are missing this should be easy to spot and fix and it has been quite a long time now.