Where is the continuity for Army leaders in regards to assignments?

A year and a half ago, NCOs and DA Civilians from HRC's Infantry branch came to Fort Campbell to talk with Infantry Soldiers and NCOs about the assignment process. They told us that we could expect to be on station for 3-5 years before moving onto another location. We were also told that certain positions, namely Platoon Sergeants, could expect to move after attaining 24 months of professional development time. However, here I am, at 18 months on station, and I'm already on orders for Drill Sergeant School, yet there are other NCOs who have been here, either in the same company, or at least the same battalion, for more than 5 years. How is this possible, and what happened to sharing the wealth throughout the Army? In almost 19 years, I have only been at a location for 3 years twice, once on recruiting duty, and the other at my last duty station. Where is the stability that we've all been looking for for our families and lives in general? What about the stability for units by keeping experienced Soldiers and NCOs for a few years instead of having a changing of the guard every year and a half?