Why is there a factor 2 when calculating the knownRouteUnicastTime

In the XBee 868 SX user manual the formula for the following formula is given to calculate the timeout between two unicast transmission on a known route:
Why is there a factor 2 included? Is it because of the acknowledgements which are sent back? However I had the feeling that the acknowledgments where already taken into account in the unicastOneHopTime parameter