When you first select the destination on a Garmin it gives you a distance as the crow flies. Once you click 'go' it calculates the actual distance and displays this on the screen.

Garmin began with ship and aircraft GPS so this initial calculation is probably down to this.

If its a Garmin then I could believe anything I have the latest model and it lies in the glove box because its absolutely useless,the mapping is hopeless with roads missing the directions are nothing but a joke does not regonise old established post codes and addresses.My two year old Becker is a far superior product what a waste of money.