In archery, it is important to keep your shoulders in line. In picture A, the individual would have a difficult time aiming because they are using their muscles to hold the bow. I realize that you use muscles to hold the bow, but in picture B, the individual is using the bones in their arm to hold the pressure of the bow, allowing for a steadier aim.
5 common archery mistakes - inconsistent stance - KSLInternationalArchery.com. This photo is a perfect example of the correct open stance. One of the most important attributes of your stance is consistency.