I noticed on this page that there is two ways to update your statistics.
If you click on the plus sign on the top menu bar., it only adds word counts to the current date.
But On this page, on the right is a column listing your word count entries, (each entry). In the upper right corner of that box is a link ” + Add a new update ” and if you click on that.
You get the option to enter the normal word count options, but also the “date” of the entry.
If we zoom in…
There we go. Now we know.