The plan is a snapshot. The market is a movie.
The annual plan ages inside the year.
A workforce plan is usually set once, against the market as it looked at planning time. That worked when roles, skills, and comp moved slowly. Now a band can reprice and a skill can shift inside two quarters, so a plan executed faithfully to the letter can still be wrong by mid-year.
The fix isn't to plan harder, it's to plan to re-read. The teams that do well treat the annual plan as a hypothesis they check against live market signals each quarter, adjusting the harder-to-fill and faster-repricing roles before the gap becomes a crisis.