Simple.
There were seven divisions amongst Spiritual Herbs – the Common, Black, Earth, Heaven, Mystical, Ancestral and Origin Grades. However, similar to the Unique Grade of treasures, the Crown Spiritual Herb was a sort of wild card.
Crown Spiritual Herbs could be of any grade, but they were special in that they were unparalleled in benefits among those of their grade. Essentially, they provided the perfect result every time.
A Crown Spiritual Herb was an Herb on the verge of evolving to the next Grade. Herbs with the ability to evolve in this way were incredibly rare, so much so that they pained Ryu to simply sell like this even though Crown Spiritual Herbs could be sold many times that of Herbs several Grades above them.
The reason for this is simple. Crown Spiritual Herbs could provide the benefits of a Spiritual Herb of higher grade without the restrictions that came with eating one.