On my D200: Top is first attempt, focusing from infinity. In fact the Beach Boys-box is in focus - ie. backfocus. In the middle is a refocused shot (pressed AF-ON again after the first/top shot). Just about perfect!
Bottom is focused from near limit - the Nuggets-box is just about in perfect focus, so this shot is frontfocused!
Conclusion: The Sigma's AF-motor simply stops too soon!