"Pre sighted in" what doesn't that even mean? I assume it means the emitter projection is centered up with the axes of the indented platform. It is like bore sighting a rifle. It will get you on paper, but the rest is up to the end user.
Irons on OEM Glocks are known to have some variability straight from the factory. There have been a few threads about it on the Glock subforum. Your dot could be dead on and irons could still be off.