If you are referring to a solid rest, the mount may have a lot to do with it.Two of my shooting buddies, who shoot 2-3 times a week, have all had the same experience. If not paying partular attention, gun mounts are not consistent.
If the buttstock is either high or low, it will give you horizontals. The three of us are used to shooting very small groups and when we start seeing horizontals, we go back to the basics and watch our mounts. I've not had the experience with verticals, so have no opinion.
