Differences often lie in the size, angle and depth the same fold can take among humans.
Early in life, the shape of the skull is very important, the alignment of its parts particularly.
For example, the fold going from the side of the eye to the top of the ears is obviously affected by their respective positions.
Most of the distinctions come from how big the fold has become along its path.
This, in turn, depends on each individual's actions.
The way your nose is twisted, the way your eyes look, the way your eyebrows are formed, ... represent years of nose twitching, eye squinting or worried facial expressions.