Because they are returning within a 24 hour period... as far as I know, the counter does not count returns within a specific time frame... I think its 12 hours actually.
If you have a visitor on monday, and they come back on wednesday, it should show as a returning visitor.
Not 100% sure on the time frame for the counter anymore though... for specifics, open a ticket in the support center.
Thanks for the reply. According to how they say the break down unique, first-time, and returning visitors, my counter is not working. I'll take your advice and open a ticket. Thanks!