For almost all websites, the ratio of unique visitors to overall visits decreases as you display longer time periods in Google Analytics.
To illustrate, let’s say that:
• your website receives 100 visits daily
• each visitor visits only once per day
• 50 of the 100 visitors each day are return visitors
If you report for a single day in Google Analytics, unique visitors and overall visits will each total 100, or a 1:1 ratio.
If you report for two days, however, overall visits will total 200, but unique visitors will total only 150 for a ratio of 3:4, since 50 of the visits on the second day will be attributable to return visitors from the previous day.
For a website that had no return visits, this ratio would not change for a longer time period. Since almost all websites do, however, have return visitors, it is normal for the proportion of unique visitors to decrease relative to overall visits as you select longer time periods to display in Google Analytics.