I am just a user, but I have researched this, and being a sociologist helps me to interpret data sets.
Both apps are not giving you all 4 sets of Non-REM sleep data. Both groups of developers know that the deepest sleep only comprises a small portion of your nightly sleep. But separating stages 3 and 4 (both deep sleep, but 4 is the deepest and shortest of all) is not possible without using an electroencephalogram: wires attached to your head to record your brain waves. Muse makes a home version, but it is not affordable for the average consumer and doesn’t look particularly comfortable for sleep, either. And I suspect it is not up to medical standards as well.
So, they have two choices:
- Combine the two deepest stages using actigraphy (and HR, breathing - whatever is available), or
- Guess. FitBit is guessing. It can’t be based on body movements, because they are roughly the same. So their algorithm tries to determine when you were moving the least. But it’s just a guess. Sleep as Android doesn’t attempt a guess. It combines stages 3 and 4. And that’s all there is to it. Plus FitBit are likely throwing a significant amount of actually light sleep in with the stage 3 deep but not deepest sleep. So they are combining stages AND guessing.
In my opinion, SaA is far more accurate for home use.
Hope this helps.