Drawing in program doesn’t match finished engraved one

Hi im using lightburn 1.4.01 and im having an issue with the size on screen vs finished product.
if i draw a 50x50 mm square its ok on screen but when finished the width is exactly 50mm but the height is 114.5mm and a 40x40 is the same with is correct but height is 91mm
i have attached a picture below of this,what am i missing here to fix this ?
Thanks