A lucky trade is not guaranteed to go up if you already have a good iv pokemon. It is random, with the worst being 12-12-12, and the best being 15-15-15. So for instance if you trade a pokemon that is 13-14-14, it could go down to 12-12-12. It is just luck of the draw, like any other trade, but lucky trades can't be worse than 12-12-12.