Are you taking into account that people retire, pass away, change jobs, get fired??
In down times, just like any business, there will be hiring freezes, layoffs (usually casual or part timers), and vacancies (positions not filled yet) removed from staffing.
But like any business, they could trim the fat and/or get rid of programs.
The Phoenix debacle was to reduce payroll staff and get things automated. However the genius who brought it in failed to test it for all the Government working hour scenarios. Thus a lot of people got screwed (ie, not getting paid).