You need to make sure it's an apples to apples comparison. Group bookings often include a cocktail party or a special dinner or something, and you need to make sure you are comparing the same category of cabin.

If the category of cabin is different, it's possible you can contact whoever is handling the group booking and see if they can extend the discount to you on the other category of cabin.

You should also speak to whoever is organizing it, they may have guaranteed them a certain number of cabins and if you book through another channel it may not count.