What's odd is that I've seen OTHER counters on s05 that are fine ... with data reporting the way one would expect it to.  It's just this one (which would be the one I've been keeping .xls sheets on!) seems to be screwed up.

Yes, the way it works is there is a program that runs every night at midnight that looks at each Flag Counter on the server and records the current number of visits as being the total for that day.  When the server became overloaded, this process took a very, very long time.  Since it actually goes in alphabetical order, your ID, starting with "Z" was at the end of the list.  The good news is that s05 is on a much more powerful machine and everything is running very smoothly now!  For your spreadsheet, I would suggest taking the average of the days that appear to be incorrect.