Its a POSITION not a move. Z-5 in Machine Coordinates would be right at the top, 5mm away from the homing switch. You can't move to the switch or it will trigger, so it stops just short of it. By default, in Grbl, the entire machine lies in the Negative Quadrant. Read Why is Grbl in all negative coordinates after homing? Or it so annoying and not what I'm used to!