Why cannot always use body parts to find out the correct measurement of an object?

Some body parts are longer than others some you might not get the exact same measurement. (here's an example). Say my arm is 2.7 feet and yours is 3.4. we might not get the same answer

Because some people may have longer body parts than other people. your arm may be (for example) your arm may be 3.7 feet long but the object is either too long or you cant get an exact measurement of that object.