Losing muscle with fat is ONE factor, but I think the bigger problem is that the foods that are consumed to cut (generally lower in carbs) are opposite of the foods needed to fuel lifts (carbs). It's an issue of muscle glycogen. No carbs, no fuel, no lift. This also varies person to person. I can maintain strength when cutting, maybe even go up here and there in a move, but I don't have, as someone else said, "optimal" lifts like this. I'm not going to but PR's on a cutting diet.