Flight path larger than the planned route

The flight that overshoots the defined survey area allows the aircraft to speed up and slow down properly to make all the images be collected while traveling the same speed. This creates much more accurate outputs. If you want to turn that off you can use the Connectionless mapping setting which doesn't benefit from the speed up and slow down and stays right within the defined polygon. 

https://support.dronesmadeeasy.com/hc/en-us/articles/217006946-Connectionless-Mapping