If I download the numbers of unique visitors per day over a certain period and total them the result doesn't match with the total value in the report. Why?
The numbers in the export file agree with the number in the graph of the report.
The problem is due to the range(s) you are using. On a single day, you may have 10 unique visitors. The next day you may have 10 unique visitors again, but 8 of them came back from yesterday. So in this case, if you range is 1 day, then each day had 10 unique visitors. If your range is 2 days, then you only have 12 unique visitors since day 2 had 8 return visitors.
When Google gives you a total number of unique visitors over an entire month, they take in to account all of the users that returned to the site. When you look at it on a daily basis, you are not taking in to account that some of those visitors might be duplicates from previous days. Make sense?
On web view Google Analytics is showing unique visitors over that date period. So if you chose one month, it will show you yjr unique visitors who came over that whole month.
However on export it shows you unique visitors per day.
The same visitors who came each day will be counted as separate unique visitors on the export but will be counted as one unique visitor on the web version.
It is simply just the range. If you set the web version to just one day you will see the export day will have the same number of unique visitors.