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”

In the Bible versus the Quran: the Mustard seed is the smallest of all the seeds on earth

The mustard is mentioned five times in the Bible, all of them are in the New Testament being not mentioned at all in the Old Testament. 
The mustard is mentioned two times in the Quran.
 
Matthew, Mark and Luke give the Parables of the Mustard Seed; they said that the kingdom of heaven is like [...]