I'm sorry if this has been asked before. I thought I had somewhat of a grip on fitness and relative effort, but this observation shakes everything I thought I knew.

I went on a virtual ride today. I recorded the whole session on my Apple Watch with the Fitness App (indoor cycling activity. The recording on Strava: https://www.strava.com/activities/11579328823

I also started a couple of different activities on Zwift and saved the one in the middle where I rode 20 minutes with the C bot. The recording on Strava: https://www.strava.com/activities/11579328823

I understand relative effort (and consequently fitness) is tied to heart rate. Since the full run includes the 20-minute segment, I do not understand how the relative effort can be so much higher for just that segment alone vs the full run. 

That I'm getting one point of fitness for this effort from Zwift seems off. 

While I'm writing this, I'm answering my question here. I'm not running Zwift with a heart rate monitor that connects to it, I only have heart rate data from my Apple Watch / Fitness App recording. 

I'll leave this here regardless because now I wonder how Strava can derive relative effort (and fitness gains) from that Zwift recording in the first place. 

I know that Fitness Score is not a measurement of my actual fitness. But I like the gamification (it's the only feature I pay Premium for) and I need that number to somewhat make sense. Any insight is appreciated.

I'll run a similar workout on Zwift tomorrow with a proper heart rate monitor connected so that the Zwift recording also comes with HR data.