here are a few potential things that i can think of:
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you progressed quickly after your FF, hence why you needed to bump up the intensity in your workouts, and the later HM reflected that. Some adaptations come on relatively quickly, like blood plasma volume, muscle coordination and things like anaerobic power.
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your FF was on a bad day. There’s probably more variability in our day to day performance than we realize, and maybe you just had a bad day for it. Again consistent with having to bump up your workouts and your HM results
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you didn’t pace the FF well enough to really reflect your strenght. Any “time trial” type test depends on pacing, some more than others. And pacing is a skill. mental toughness helps, but just doing the tests help as well.
bottom line is, i think your best for validating your numbers is how you feel during the workouts.