No more than 1%, or 2 pounds, or whatever?

People in weight loss subs are always saying it’s dangerous to lose more than 2 pounds a week or more than 1 percent of your body weight. But when you ask why, they just tell you to Google it. The 2 pounds rule doesn’t make much sense since it doesn’t account for how much someone actually weighs. The 1 percent rule is a little better but still feels off. Obviously someone eating 200 grams of protein and getting enough magnesium is going to have a very different outcome than someone eating just boiled potatoes. Is there an actual explanation behind these numbers, or is it just one of those overly cautious disclaimers like "this toaster is not for use in the bathtub"?