top routed , clicks fine and all the cable is in the right place , just when it clicks it moves at least 3 gears , as i get 2 clicks and wont go a third as its fully across the cassette ??
sure its a 600? Possibly it could be an esp shifter?
I have also been fooled in the past into buying a pair of shifters which both turned out to be for the front mech, some of the front shifters are not indexed and so have 7-8 clicks. A code on the side can reveal this.