There are some serious misunderstandings about laser sights going around.
First, none of the lasers can be mounted coaxially with the barrel (looking down the barrel), so there is always parallax error between the laser path and the bullet trajectory. It makes no difference whether the laser is mounted below, to the side, or a little of both: there is only one point where the two paths intersect.

When adjusting the laser, there are two common choices:
1. Pick a distance and sight the laser to POI at that distance. There will be an error at all other distances, and the error will increase with range.
2. Adjust the laser so its path is essentially parallel to the bullet trajectory at short to medium range. There will be an error at all practical ranges about equal to the distance between the bore axis and the laser. Although this sounds worse, many find it easier and more predictable in use.

The preferred place optically for mounting the laser is on top of the slide, but that would take away the iron sights, which are needed when the laser quits.

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Science plus Art