When in the Audience > Overview and selecting data for one specific day (4/20), I get a Unique Visitor count of 1,392. When I select the data of another specific day (4/21) I get 942. But when I get the data of the two days together (4/20 - 4/21) I get a Unique Visitor count of 2,252. The math does not seem correct as 1392+942 = 2334 != 2252.
What could be causing this?