The night is set by the household, not by the sum
In almost every house the night falls at the same time almost every day, because around it there is a dinner, a bath and parents who would also like to go to bed.
napnap treats it that way: bedtime is the median of the last two weeks of recorded nights, and it does not drag when a nap runs long. If there are no nights recorded yet, it uses the hour you put in settings.
Median and not mean, and the reason is specific
In a house where the four-in-the-morning waking sometimes gets recorded as two sleeps, half a dozen split nights drag a mean and do not move a median.
The same goes for how long the night lasts: it is measured with the median of the last ten, and separately from the naps. Measuring a night against the average of the naps is what made an app announce at four in the morning that they were long overdue.
And that is why its margin is different
When a future day is forecast, each nap carries the uncertainty of the morning plus that of every window before it: the third is known less well than the first.
The night is not like that. Being anchored to the clock, it carries its own margin and not the accumulated one — which is the difference between telling you «bed around 20:00 ± 20 min» and «± 3 h», which says nothing at all.