I'm not looking at my X0 shifters now, but from what I remember, you can loosen a bolt and adjust the lever's start position in its arc of travel. I think that the little marks are there so you can adjust both shifters to the same position.
I set mine so I don't have to reach so far back with my thumb to grab it. That's what I disliked about the X7 shifters.
Salt Lake City 2012 Specialized Stumpjumper FSR Comp 29
2006 Specialized Allez Expert Double