What GPS signal does the mobile unit use? If not hooked to some form of RTK or high level then the boundary is/was moving slowly all the time on you. 

Also, if set at zero you were straddling the boundary edge while driving? Most fields here have fence posts or trees or something on at least one side so a lateral offset is needed to be able to drive cleanly while still drawing boundary line where it needs to be off to the side 

If you have no poles or trees or anything you could get away with zero offset side to side