Inconsistency in intensity assignments for activity?

[NaomiE]

I have a Fitbit One. Usually, I can expect to see low intensity during quarter hours when I walk less than 500 steps, moderate intensity when I walk between 500 and 1500 steps, and high intensity ("very intense") when I walk over 1500 hundred steps. I don't track activities manually or use gps on my phone when I walk, and since the One doesn't track heart rate or (to my understanding) factor stairs or other inclines into exercise intensity, I should see the same results for the same number of steps in the same number of minutes. Lately, however, I will see variation within a thirty minute period. For example, today I had one 15 minute period during which I walked 1,735 steps, which was marked as "very intense" (exactly as I would expect). In the 15 minute period directly following that, I walked 1,665 steps and my fitbit classified that as only "moderate," despite the fact that on many other days I have seen anything over 1,500 steps per 15 minute period classified as "very intense." The two periods in question today followed one another directly, involved the exact same activity in the same location (continuously walking around the same loop indoors, with no fans or stairs), resulted in very similar step counts, and were classified as different levels of activity. Why? This is both internally inconsistent (the same activity at very nearly the same pace in the same conditions) and inconsistent with the normal way my tracker classifies "very intense" activity (over 1,500 steps in 15 minutes always counts as "very intense"). Does anyone understand why? (I have already read all of the help articles about active minutes which come up when you search this topic.)

[AlejandraFitbit]

It's great to see you around @NaomiE. Thanks for all the information provided. I recommend checking this post where @SolangeE is providing a good explanation about the intensity on the activities. 

I hope this helps, let me know the outcome. 

[NaomiE]

The explanation provided in the linked post is that a body gets acclimated to the same workout or intensity of effort over time as it grows stronger. If I were describing a situation in which I did the same exercise routine daily for weeks or months and saw a gradual reduction in how many active minutes I racked up for that routine, I would get it. However, in the situation I described (pretty clearly, I think) in my original post, the two 15 minute exercise periods are happening within 30 minutes on the same day- so no, it doesn't really explain the discrepancy. The other thing is that the number of steps in 15 minutes required to push me into "very intense" territory hasn't risen overall- if I do just one 15 minute period where I step more than 1,500 times, it gets counted as "very intense," except for sometimes randomly in the middle or end of a workout where the change in steps/distance/calories burned is too low to explain it (sometimes the period counted as "moderate" even has *higher* counts than the one that was counted as "very intense" in the same continuous workout!). If anything, it seems that a person gets more fatigued as a continuous exercise goes on, and has to expend more effort to maintain the same intensity of activity. If the same exercise at the same intensity got easier as you racked up more time during one continuous workout, I imagine marathon running would become accessible to many more of us than it is.