A single visit can span across two days. For example, consider a visit that starts at 11:30pm on October 23rd and ends on 12:15am on October 24th. For the month of October, you'd see one visit. For October 23rd, you'd see one visit; for October 24th you'd also see one visit. If you add those two together (which you shouldn't) you'd get 2 - which isn't an accurate measure of how many visits you had in the month of October, in our example you only had 1.
Same as Andrey's answer - this applies to both Unique Visitors and Visits.