Nice spoiler... thanks for ruining it for me..
That's more lore than the actual in game story I believe. The DB is on its last leg with skyrim's charter being very rag tag, thrown together, last little resistance. I am not surprised that their armor is obvious, they shouldn't be wearing it in the open, stealth in, costume change, whatever you need to do. If, and it is a big if, they leave any witnesses, they want people to know it was them, and I figure people do know anyways. But Ah my main question was, why the hell is the armor bonus on the chest piece 50-100% poison resistance? Why would an assassin be poisoned? I'm the one doing the poisoning. Where do you even take poison damage? I usually wear the thieves guild chest for the carry bonus, haven't gotten all the way through yet.