Thank you for your question. The reason for different rulings despite shared sources is because of intricate differences in the methodology of deriving and applying those rulings. These positions are taken by Jurists over years of careful study as their implication is that rulings will be different. To understand this further a person will have to study the principles of jurisprudence as well as have an awareness of the modern discussions in fiqh.
It is possible to follow the rulings of more than one Marja if they are all at the same level and if there is no possibility of identifying the most learned Jurist among them.
May you always be successful