Wort, when prepared properly, contains much more nutrients and goodies for the yeasties to feed on. It also contains sugars that are easier for the yeast to break down. Because of this Wort ferments out much quicker with no need of additional nutrients or O2. Must, on the other hand, lacks the nutrients needed for the yeast. Hence the multiple Nutrient additions. As the yeasties try to break down the sugars in the Must, they burn through O2 and yeast nutrient as they start to build the huge colony needed to ferment the Must to completion. So with each nutrient addition, you also want to replenish the O2 supply. However, once the yeast ferment 50% of the sugars in the Must, that's when you typically stop adding nutrients and more importantly O2. Any additional O2 added to the Must after that 50% point will greatly increase your risk of oxygenation. That's why your nutrient additions typically end after about 3-4 days. Hope this helps!