User blog comment:Pinguinus/Kidneys!/@comment-24121492-20131227090114/@comment-27270572-20131227112822

Maybe he dies there in the future and so the grave is still there as he never saw which face it was that died. Or maybe both versions happen like the Doctor dies in one loop of time and in another he survives but both of the times will both happen like in the series 6 finale where River changed time and Amy could remember her killing the Doctor and not killing him. I am probably just rambling but I thought I would try to give a timey wimey explanation.