The deputy is always the one who runs operations, so that isn’t really unusual, it’s just the things we know about this particular deputy AG that make it an issue.
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Normally, a strong leader would delegate operational responsibilities over to his/her second in command so that the leader can focus on organizational goals, policies, priorities and general direction of the agency. However...in this case it is highly probable that Rosenstein is exerting a strong influence over the weak and indecisive Sessions.
Sessions is a politician, not an executive. Rosenstein is a bureaucrat and a partisan manipulator. The clueless Sessions is putty in his hands.