People, don't forget that the phones "compus" (magnetic sensor) is calibrated to measure Earth polarity through magnetic alignment. It will indeed vary along any other magnetic materials you present close to your phone, but will not measure by any means "correct numbers" of magnetism if you are thinking in using it to measure "for example" how much strong a magnet is.
So, don't go back to the manufacturer of things you buy and ask them for a refund because an app on your phone said the magnet was not as strong as it was saying in the box. LOL