It is controlling for variables. They are trying to determine the role of macronutrients on food partitioning and, ultimately, weight. One can argue they should have used all women instead, but I think the challenge there is women have a more varied hormonal environment due to menstruation[1], which, again, adds a variable. As I understand it, fat partitioning appears capable of change during menstruation.

Of course, then we run into a problem that the results may not apply to women. Nutrition science sucks.

1. http://itsthewooo.blogspot.com/2014/08/food-reward-hypothesi...