i'm not sure that info is accurate. I've been quilting quite a while and that is the first time I ever heard that stitches on the underside are counted. I'm not sure where to research the information, but i was always under the impression it was measured on the top of the quilt only. There are people who get 12 stitches to the inch on the top and you never hear them saying they get 24~
Rather than focus on the number of stitches per inch, I would just work on keeping stitches even. There are many factors which affect the closeness of stitches, including thread count on the fabric and thickness and content of the batting, so it is not necessarily fair to compare one quilter to another on the stitch count. Just strive to do your best and it will be fine.
Lisa