I could see how that could work. As the object moves it keeps triggering trips and so the camera goes to each trip point, whereas with a single trip as a box outer, once the outer is tripped, the camera receives no further trip input and so is free to roam based on it's tracking of the object, which may be poor!
Why Dahua don't implement the object tracking we see in even simple software these days where an object is identified and a green box drawn around the object of interest which then becomes the focus, and the object then tracked is beyond me. That system seems to work quite work in the software side for tracking even people. However, I can see how giving the camera lots of trip inputs may keep it on track as an alternative.