"cable stretch" is a generic term to describe breaking in of new cables/housings, etc. especially in a new bike.
generally during a new bike break in period adjustments to the brake and gear cables will need to occur.
when i perform a new bike 'tune up' i generally have to remove the slack from the cables...it's usually not much but even a little will affect precise shifting.
i check everything from rear der alignment...hanger could be bent a bit...limit screws/alignment...front der position and adjustment with the cable loosened to the limit screw for the easiest rear gear is correct and the spacing between the guide and big ring is correct as well as the cage angle then attach the cable and adjust as needed.
it does happen and is part of a bikes maintenance.