1. Visits simply mean visitor activity within 30 minutes. So if I went to
your site, I'll be counted as 1 visit. Let's say you have a page called A
on your site. I click on that page the counter will be 1 Visit/1
Pageview for A. I click on refresh on my browser for page A, that'll
still be 1 Visit, however you'll have 2 Pageviews for A.
Now if
within my visit session, I became idle on your site for 30 minutes and
on the 31st minute, I clicked on another page on your site, let's say
page B, your visit counter will be 2 Visits/1 Pageview for B.
2. Unique Visitor works for a time span
Suppose you have 100 unique visitor for the month of may and 120 unique visitor for the month of june
Now
changing the time span in calendar from 1st may to 31 may it will show
100 and when you change to 1june to 30th june it will show 120. Now, it
may have happened that A visited your site on 12th may and again on 20th
june that means he visited againn. So while making monthly report for
may and june respectively A will be shown as unique visitor for may and
for june.
Now when you change calendar date from 1 may to 30 june.Then A will be counted as unique visitor only once.
Thats why you see the unique visitor result decreasing while setting the longer time span.
Best idea would be generating monthly report.