They aren't placebo. For example the strength or agility. Those are what they are, depending on your muscle mass+composition, motor coordination, etc, you can't overcome your limits through placebo effects. The stamina as well, in fact. If your current fitness allowed you to keep running for 4km, but the cursed item with a stamina effect allowed you to run for 8km, there's simply no way to explain it as a placebo effect. It's plain impossible without magic kicking in. However, that doesn't mean the stamina wouldn't be depleted when the item is removed, much like as if you didn't have the item at all and had used up all of your strength.