Imagine the process of trying to describe features in detail so someone else can understand them enough to draw them.
Were the eyebrows curved? Where was the curve? Mid, inner edge or outer edge? Did they extend past the edges of the eyes? How high above the eyes were they? Were the brows pronounced or more even with the forehead?
Imagine the poor artist trying to draw something with limited detail. What do you put in there when you don't have good detail?
Sketches are more to highlight the major details so it resembles someone, likely in order to trigger other memories, or confirm other memories. "Hey, that sort of looks like my creepy coworker who also wears a red cap all the time, and that was the day he took an extra long time to do a delivery."
I believe the sketch artists have renderings of different type eyes/brows/noses etc to help. Basically point to a similar eyebrow then artists sketch.
ETA- I agree under pressure/fear etc one can be off on their description. Witness's identification of a suspect I think has been way off before.