I'm just saying if one has RTK or some other accurate signal you match them ideally. If that's 40 or 39 or 20 or whatever. 

Yield is calculated using distance traveled, swath width, grain flow, etc. If swath width is incorrect....yield is. If you exaggerate the numbers a bit to make it easy to see....say you had 45' in as your "mapping" or head width but you were actually steering on 40' spacing. Your map will show overlap with the neighboring passes and you will have falsely low yields since it's saying x amount of grain accounted for 45' wide area instead of 40. 

On the other hand...you can falsely inflate yields by having too narrow of a head measurement in.