This is because carrots don't contain Vitamin A but a carotenoid called beta carotene which is a “provitamin” — a substance that can be converted into a vitamin.

因为胡萝卜所含的并不是维生素 A，而是一种叫做β-胡萝卜素的类胡萝卜素，它是一种“维生素原”，一种可以转化成维生素的物质。