Tracking ip route reachability will only look for the existence of the PREFIX/MASK combination in the routing table, while metric threshold will consider the tracked object to be up if the scaled metric of the route is less then the UP-TH, and down if it is more than the DOWN-TH. The value of the threshold si 0-255, so the metric of the route will be scaled when compared.
The value used for scaling is configurable with the following command: