Answer
Because of lateral inversion phenomenon in mirror when someone in rear mirror or side mirror sees the ambulance behind them they can read it correctly so it is written in that way. When viewed by a driver through a rear-view mirror, the right-left reversal of the image formed by a plane mirror will make it read "AMBULANCE".
Work Step by Step
Because of lateral inversion phenomenon in mirror when someone in rear mirror or side mirror sees the ambulance behind them they can read it correctly so it is written in that way. When viewed by a driver through a rear-view mirror, the right-left reversal of the image formed by a plane mirror will make it read "AMBULANCE".