A girl crosscountry skis.
A girl is wearing a cap.
Not all skiers wear a cap.
It is not necessary the girl should be wearing a cap during skis.
A girl may be clothed or a girl may be nude; a girl may wear a cap or a girl may not be wearing a cap; while it is almost entirely likely girl is clothes while skiing cross country, it is not certain. Since there is more than one possibility with a cap, it is not certain that girl is wearing a cap.