Why does the Bible incorrectly state that the mustard seed is the smallest?

there are many other seeds that are smaller, for example orchid seeds are almost microscopic
Matt 13:31-32: “the kingdom of heaven is like to a grain of mustard seed which…is the least of all seeds, but when it is grown is the greatest among herbs and becometh a tree.”
*bonus error: mustard plants are not ‘trees”