Love the description from Susan Fletcher- A Threaded Needle,we really had to share with you
Sashiko thread is not made in strands like embroidery thread, it is made of fine threads twisted together to make a single thread (yarn). You use the entire strand when stitching with it. This difference does matter. Embroidery thread will show the separating strands in the longer sashiko stitches, while sashiko thread will settle into the fabric as a solid little "rice grain like" stitch.
Sashiko thread is made with a rougher/grippy texture, and has a flat rather than shiny finish. These characteristics allows the stitching to plump up and "cosy in" to the fabric in a manner that is distinctive to sashiko stitching. This effect increases with repeated washing of the stitched fabric, increasing its beauty over time.