algorithmic modeling for Rhino
Further testing reveals that when you use a negative number for the index, it causes inaccuracy. Also as you approach the wrapping other inconsistencies are caused.
I don't see it. It's working as it should be. If you look at the appropriate tree, the square, the hexagon and the circle are all in column index=2 (i.e. the third column) and item index=5 (i.e. the sixth item).
I attached a file that helps you investigate the layout of points within each tree.
Perhaps the behaviour you were expecting happens when you wrap both the paths and the items?