Magnets manufactured today may have closer tolerances than products made 40 or 50 years ago. Over time the magnet can be altered by age, heat, shock or other outside electrical elements such as transformers or other magnets. Companies try to keep tight tolerances when being manufactured but some are not always kept. When I say a magnet ages, I am saying it can become weaker because of the realignment of the magnetic domains within the magnet. Some think a magnet can’t become weak but they do. Why do you magnetize a magnet anyway? To alien the magnet domains within the magnet. You can reverse the polarity of a magnet too as some think you can’t.