Because they are made like a fishing rod, carbonfibre-mats get wound around a inner-form. at one point, where the mat touches or overlays itself again there is a seam. At fishing rods you need to take this seam to determine they way it will bend, BUT (and thats a big But) at fishing rods it just is an overlay. At carbon arrows it gets filed /polished down to the normal diameter. Nevertheless a slight seam can stay because of the fibres being more dense there. I even know people who determine this seam and fletch their carbons like woodies along the grain/seam.
I think i wont be able to shoot that good to make out a difference in years.
(Hope this complicated thing is understandable in my english, if not, sorry)