Asked a similar question before and never quite got to the bottom of it.
My take is that there is an assumption built into the plans that you will get fitter (whether that’s FTP, endurance, VO2, etc depending what the plan is targeting). I.e. If you took the FTP test and then jumped into week 5 of a 6 week plan, or week 7 of an 8 week plan, you would likely fail to complete it, or else completing it would dig you into a big hole. Which is kind of the point - the plans are designed to be progressive and make you stronger at the end than at the beginning.
In practical terms it means that if you’re not progressing during a plan for whatever reason (work stress, poor recovery, interruptions from injury or sickness, or maybe you’re just stuck on a plateau) then you might find the latter weeks of a plan are really tough and may not be possible. I’ve had this happen a couple of times, and have had to either adjust intensity to get through some workouts, or replace them with a slightly easier version. Have also had the opposite when coming back to cycling after a break and my fitness is shooting up much quicker than is built into the plans and I’ve had to adjust FTP up between tests because the workouts are too easy.