Material Witness

In the fabric shop, the curtains are neatly arranged by style. The floral-patterned ones are in a section marked “Floral,” the plain ones are in a section marked “Plain,” and the striped ones are in a section marked “Striped.” But one pair with vertical blue stripes is not in the “Striped” section. Why not?

Clue: They are perfectly normal curtains and not special.