Meat contains lots of water. While you're roasting it, that water evaporates in the hot oven. At 180 degrees, almost all of the moisture in the meat is gone, so you have perfectly dry, perfectly tough meat. That's the cooking that continues after you take the roast out of the oven. It easily can add 10 degrees to the temperature of meat and turn a perfect medium into well-done shoe leather.
Nothing causes more angst.
It is impossible to give the exact timing for a roast. Use a thermometer and if you're not sure, cut the meat and peek. Yeah, it'll release a tiny bit of the juice, but that beats overcooking or serving a roast that's still cold in the middle. Place pork in shallow roasting pan. Heat oven to 400 degrees.