I'm only about 70% on this, but I think that the range from your headshot kills also sum up together for those 250m marksman ribbons. I played a match where it was not a very big map and I guarantee that not even one of those kills was a 250m headshot. I had 25 kills in this match (most those kills were headshots) and 5 or 6 marksman ribbons. So if you get a 280m headshot, you'd only need a 220m to get the next marksman ribbon. That's my guess anyways.