Zone Minutes inconsistencies

[Carlocom]

I normally go for a daily walk that lasts about an hour and usually gives me about 40 minutes as "Active Zone", based on my heart rate. However, sometimes this value is extremely low, despite me following exactly the same pattern, as shown in the picture below. 

I would like to understand if I'm missing something or if it might be a bug/glitch causing these inconsistencies.

Thanks

[USAF-Larry]

@Carlocom, the difference is most likely because of your heart rate, and the duration at the elevated rate.  One one charts shows the Average heart rate of 122 bpm, while the other shows and Average heart rate of 111 bpm. The lower heart rate was a result of the slower pace in one walk compared to the other. Check out the help article, What are Active Zone Minutes or active minutes on my Fitbit device? 

[Carlocom]

Thank you, I guess that's it. Just seemed odd since even pace is very similar and difference in minutes is quite significant. I'll just walk faster then lol