That's because when you're on the elliptical, or doing rope pulls, or running around the neighborhood you're training for endurance, meaning that your lungs are going to be way more active for a longer period of time than they are when weight training. (Remember: Fat is oxidized or "burned" through heavy breathing.) Conversely, training with weights in the traditional sense means that you're not training for endurance, but rather for hypertrophy (muscle building), so it would make sense that you would lose weight (both fat and muscle) through endurance training, but you would maintain or even gain weight through resistance training.