Rune of the Thief
You need to purchase all 6 to get the full amount of bonuses.
I had the same thing. The reason I think is that you probably have this set equiped in pvp (heart of the mist) and thats why it shows up as u having 6/6.
Grinz is correct. If you go and replace the runes in your HotM PvP gear, you’ll notice this will change. The system is a little buggy in that it accounts for all the runes attached to your character in BOTH PvE and PvP even though those two areas of the game are separate.