Suppose your true rate holds at10 per 28 days, forever.
You still measure 8 or fewer about 33% of the time, and 14 or more about 14% of the time. Roughly 70% of periods land between 8 and 14. Our "14 โ 8" sits inside that.
โ ๏ธ This assumes events arrive independently. Real traffic clusters by weekday and posting time, so swings run wider, not narrower.