Don't know why that is, but it is true. When I came in to Civil Service I came into a ladder position that was set up as a 7/9/11. No 8 or 10 in there. I think it just depends on how the position description (PD) was originally written and classified. Each PD has a specific format with knowledge, skills abilities, scope of work, contacts, supervisory controls, etc. Each position series (or code) has a classification guide, sometimes multiple guides, that are used to grade the PD. Just depends on how the PD grades out.