Did you observe the machine’s behavior while it was working on that last column? Any suspicious clicking noise on the X motor ? Are you able to jog manually to the X position where that last column should have been cut, and check whether anything happens at a mechanical level that does not look/sound quite right ?
The actual X/Y travel that people get is slightly different from one setup to the other (depends on Z-axis type, dust shoe, dust shoe ears, etc…)
You may need to tune the dimensions in the wasteboard design file to suit your machine.
The other/additional possible culprit would be a loose set-screw on the X motor pulley, it is possible that when reaching the last column, extra force is applied to the X axis, which instead of lost steps on the motor (clicking noise) could result in the pulley slipping on the motor shaft is setscrews are not installed/fastened properly.
Julien knows his stuff. Maybe you should return your machine to x0 y0 and see if it is still where you originally set it. You may have lost steps and that is why the rows are off. I made the Myers spoil board and it worked fine. You most likely have a mechanical problem or your carriage is running into something and losing steps.
since this is a new machine the set screws are #1 culprit and #2 you have hit something or #3 your belts are not tight enough.