Pretty simple. Skjei already put on 10 pounds of muscle while Nieves did absolutely nothing in the off-season to get strong. I like Nieves but he failed miserably in the off-season to get in better shape. Kristo over Skjei is laughable.
Where did you read Boo failed to gain weight? And gaining weight is not necessarily the same as getting in better shape.
Kristo is further developed and a very good prospect in his own right. Some people may value that more than pure upside. They were picked less than 30 spots apart overall. I wouldn't rank Kristo over Skjei, but it's not laughable. Different people use different criteria in ranking prospects.