-mounting points vary considerably between models.
-certain manufacturers are significantly forward or behind the mean in many models.
-part of this is due to design, part is due to different corporate (national, some may argue) styles. part is intended use, though where this is likely to be broad you will often see multiple marks.
-skiers are not all mechanically similar.
-some skiers decide on some skis that they'd prefer to be mounted ahead or behind a particular manufacturer's interpretation of the "right" midsole mark.
If you tested the ski with a midsole mark setup and liked it, go with that. Knowing if you want to go forward or backward from there is something you really need to learn by experience.
a friendly reminder to others who find this thread: while very unlikely to be an issue with a mass produced ski these days, every year I come across some skis where the printed marks are inaccurate. check them for sanity vs. common formulas or other skis.