Why is the Find my Phone app showing my iPhone to be in a different location than where it is?

I have my son's iPhone registered on my Find my Phone app. but it's showing his

phone to be in a different location than it is - I know as I've just called him at his friends

house on the landline but its showing him to be about 1/4 of a mile away from where he

is, does anyone know that would be please (he had his iPhone on him)? Thanks