Nearby stats show the median of the last seven 7 days, current day included, i.e. it is the 4 value of the seven last days, ordered by size. So if the new value and the dropped value are similiar, the median and therefore the shown values does not change.
No problem, it has been discussed here before
It is not very intuitive and not really explained on the stats page.
The last day would probably be good enough for most people, but I guess FA wanted to have an “average” value without actually calculating and storing an additional value?!
It’s not about what’s more expensive to compute (there’s no real difference). A median is arguably more useful than a mean here, because weekly data has regular outliers (the weekend)