Docs said girls typically stop growing around 19-21 except for few rare cases. what would cause those few rare cases where height not achieved by then?

Genetics, hormones: Your genetic make up and certain hormones that may fluctuate can cause late growth spurts on rare occasions. One of the most famous cases was David Robinson who went to the Naval Academy, which does not take tall people because of Submarine regulations, then he grew almost a foot or more in College. They kept him of course, he broke school basketball records, and went on to play in the NBA.

Odd data: I'm not sure where you get your info but most women in the US end their linear growth by 16-17.The hormones that support pubertal changes and menses cause the bones to accelerate,then close their growth activity, which averages 2.5 inches after the first cycle.Those whose genes cause a delay in puberty have growth centers that remain open longer, a support growth later.