Onefinity Foreman CNC problem

Hey so I’m using the Onefinity CNC with Carveco maker and programmed a pocket to surface my spoil board (waste board). I had programmed it to cut 1/8 off the top but after I had zeroed everything (x,y,z) it had cut an extra quarter inch deeper than it was supposed to! I tried a few things and it didn’t fix it. The z axis still says that it’s cutting 1/8, so I tried to zero the z a quarter inch higher and it seemed to work pretty close. But I was wondering why the machine thought that the zero depth was a quarter inch deeper than it was supposed to be?