The angle of the break over point where the cable and string roll over to attach to the post is pretty steep which tends to cause the seperation you are talking. Plus the matthews stock strings leave much to be desired IMO.

Usually a good aftermarket string will take care of it. I had no separation at all with Vapor Trails', then switched to a Little Jon and had just a little but not much seperation. They say anything under 1/8" isn't cause for alarm, but alot of times it will get bigger over time. You can push the serving back together with your thumbnail and serve over the ends of the existing serving about 1 -2". That's supposed to hold it together. Some also put super glue on the serving ends too.