Dealing with a larger offset when cutting a larger object, try calibrating your machine accordingly () now you will have to do the last couple of steps a few times to get your machine as accurate as possible.
Also if this does not correct your issue, try looking at the tightness of your x/y/z - axis chain, bearings, and also your sprocket/rod, the chain must be viewed as a bicycle chain if too tight it might snap when the machine moves but if to loose then it will be off. The rod is not totally flat where the set screws are suppose to hold down the rod, so sanding it to a flatter surface section might fix the offset.
Click the link to respond:
Can your cnc's get within 1/10th of a human hair in accuracy while cutting?