By default, Suunto Ambit3 Vertical takes both elevation gain and loss into account when measuring distance. This gives you a more realistic distance measurement when you are exercising in hilly or mountainous terrain. In this scenario, 3D distance is greater than 2D distance.
In most cases you can leave this setting as it is. However, if you are using 3rd-party software to analyze your tracks, check how the track distance is calculated. If the calculation is using 2D distance, you may want to deactivate 3D distance in your watch.
To deactivate 3D distance: