A brief explanation of the frequently asked question : what's the difference between Absolute Unique and Unique Visitors in Google Analytics? For many, these two distinct measures are used interchangeablely; a dangerous practice. In fact, largely different results can be produced by both measures when analyzing the same site in the same time frame.
Most simply put, Absolute Unique Visitors are first time visitors to the site, whilst 'Unique Visitors' are first time visitors to the site within the selected date range.
In more detail, Absolute Unique measurements take into account whether or not the user has been to the site prior to the start of the set time period as defined in the date range box. The Absolute Unique Visitor report counts visitors to your website (counting each visitor only once in the selected date range) and is fairly limited: you can’t use Advanced Segments on this report (one of the few where you can’t). Visitors who have returned are still only counted once.
Unique Visitors (or visits) represents the number of unduplicated (counted only once) visitors to your website over the course of a specified time period. A Unique Visitor is determined using cookies.
The two metrics do differ in the way they measure visitors, so the numbers won’t match up exactly. In fact, Unique Visitors is actually more accurate than Absolute Unique Visitors, even though the name suggests otherwise. Unique Visitors will usually show a slightly greater number (usually around 5-20%). Since Unique Visitors is the more accurate and flexible metric, Absolute Unique Visitors is slowly becoming obsolete.