Declining Cardio Fitness Level despite faster pace at same HR
i did a 3 mile walk at 13:57 pace and average HR of 121 and it said my fitness level was 36.3. A week later i walked the exact course at 13:45 at the exact same an average HR and it says my fitness is only 35.7. Today I did the same course at 13:26 and the same HR and it says my fitness level is only 35.1. How can my fitness level keep declining if i can go faster at the same HR?
[Re-Titled by Moderator]
Apple Watch Series 7, watchOS 11