The z is fine. Its the y or say north/south direction. Best way to explain it, is this. IN mach you have your display screen. It shows you your table, or what it thinks your table dimensions are. So you have your z set in the front right corner, as if it was home. It should show your work in the left corner on the display. But for me, it shows the z in the back left. When I jog the z back by hitting my keys, the cross hairs on the display show it moves forward on the display screen. But it cant gi anywhere because I am at my limit on the front of the table.
So it thinks my workspace is in front of the Z, and not in back of the Z. hope this helps