ll peel and stick films have stretch rate, millimetric (1-2 millimeters per 8-9 feet tall). The stretching rate of your wallpaper will be exactly the same for all panels. When aligning wallpaper panels during an install you might notice the top matches perfectly but the bottom is 2 mil. off, just trust the process, as you peel the backing paper you will see the misalignment disappear. When force is applied while removing the backing paper, more stretching rate will be experienced.  The backing paper needs to be removed with both hands in horizontal and parallel position, pulling even with the same tension on both sides, this way you will feel a slight resistance but never the need to pull hard. If you feel the need to apply force, then you are doing it wrong. Never pull the backing paper with just one hand, or favoring left or right directions.