Greetings,
Re: 1. Observed from the video with the below reasons, "the 4" walked a longer path. This happens normally (though i guess subjectively normal, or abnormal ) when area is filled with blocking objects(i.e. many walls/palisades/narrowly passable terrain/player(atker/defender) units). It is a common mechanic to defend against rams/elephants, forcing attackers to pathfind the long way, or even make them circle, with buildings/units/terrain as blockers.
"the other 4" has a different formation than "the 20."
The bottle-neck is on an incline, with a certain traversal width, and subjectively a high angle of departure with the objects, limiting the bandwidth, and eliminates a passable route.
"the 4" group's path is separate from "the 20", calculated on a different frame(?), the shorter path is path-able for "the 20" and impossible for "the 4".
they both eventually arrive at the destination
Regards,