I think Adam miscalculated.  60/40 seems like you are only giving 10 more points to one person, but it's actually 20.  I bet he wish he had done 55/45. Edited to add that I think whenever a coach wants to favor one of their team members over another in the semifinals, they should go with 55/45. That way, they can…

I am from the future.  Juliet will clinch her win in the finals with a Janis Joplin song.

Yes, this is the basic problem with the show's format.  It ultimately can pick the single best singer, but in the process of getting there many of the top contenders get eliminated earlier than they deserve to be.