They are not always right... I think they just looked at GGGB first week sales and Rated R first week sales and made up this prediction...
They don't look at past numbers. They look at data from stores that send in sales data daily and then make an estimate based off of those numbers. Besides, 180k isn't that bad. Pop albums just don't sell anymore. Katy barely got over 200k, and she had two smashes to her name when that album came out.