Most of the autorefractor (machine that measure the power of the eye) has some deviation. The eye itself has a crystalline lens that help to focus near. While autorefractor measuring your eye power, the eye might start to accommodate(the lens inside the eye start to focus). This will cause a high power reading, commonly is short sighted. The younger you are, the higher the chances that you accommodate during the examination, therefore create a false result. The best way to reduce the error is by testing the power in the examination room (see link on examination room requirement).
The best thing for an autorefractor is to measure a physical parameter that do not change, for instance the curvature of cornea. So don’t relay too much on machine to measure the power of the eye.
