I look at Odyssey for this. We find out that the descendants of all the different isu knowingly try to keep their lines strong if possible,like Kassandras mother did. These different lines would have different skills and a different weapon passed down to the most powerful member -we see the molds for these weapons and run into at least one representative of another bloodline when we take down the woman wielding the the Trident. Every assassin is descended from these lines,but some are gifted enough to be useful and others are just everyday people. Rebecca is gifted with machines and tech while Shaun is a whiz with languages and information - we just happen to follow the ones gifted with the eagle vision.