this is a calibration problem. We have some people who calibrate once and are
spot-on, and others who try many times and still have accuracy problems. The new
holey calibration approach seems to be more accurate, but is not yet in the
mainline Ground Control and Firmware (you either need to use webcontrol which
supports it, or the separate GC/firmware)
you can go through the calibation again and try to be more precise, or you can
try the holey triangulation calibration and see if that ends up being more
accurate for you.