Why does my Apple Watch show different Functional Threshold Power results?

The FTP calculation in the health app is confusing. One day, I do a 70 minute ride on a spin bike at a fixed tempo and effort with an average heart rate of 124 BPM and an average power of 144 watts and the health app shows my FTP as 188 watts. A week later I do an 80 minute ride at a fixed tempo and effort with an average heart rate of 124 BPM and an average power of 147 watts and the health app shows my FTP as 182 watts.

How does FTP go down when power and time both went up and heart rate remained the same?

[Re-Titled by Moderator]