FAQ: Visitor Totals Don't Add Up
When I add up the number of visitors on each day of the month, and I compare it to the total visitors for the month, they're not equal. Why not? Also, why doesn't the sum of visitors on subpages/subdirectories add up to the total for the directory, and why doesn't the sum of visitors on subdomains add up to the total for the domain, etc.? Why are there dashes () for the visitor totals?
Short Answer
Because "visitors" is the number of unique visitors, a visitor who visits every day will show up as a single visitor in each day's visitors count, but also as a single visitor for the whole month  not 30 visitors! Therefore, simple summation of visitor numbers gives meaningless results.
Long Answer
We get this a lot as a bug report, but Sawmill is not counting visitors wrong. "Visitors" in Sawmill's terminology refers to unique visitors (see Definitions of Numerical Fields). So:

The total hits in a month is equal to the sum of the hits on the days of the month
and 
and the total bandwidth for a month is equal to the sum of the bandwidth on the days of the month
and 
and the total page views for a month is equal to the sum of the page views for each day of the month
BUT 
The total number of visitors in a month is not usually equal to the sum of the visitors on the days of the month.
Here's why. Suppose you have a web site where only one person ever visits it, but that person visits it every day. For every day of the month, you will have a single visitor. For the entire month, too, you will have a single visitor, because visitors are unique visitors, and there was only one visitor in the entire month, even though that visitor came back again and again. But in a 30day month, the sum of the visitors per day will be 30, or one visitor per day. So though Sawmill will correctly report one visitor that month, it will also correctly report one visitor per day.
If what you're really looking for is "visits" rather than "visitors" (so each visit will count once, even if it's the same visitor coming back over and over), then that's what Sawmill calls "sessions," and you can get information about them in the Sessions Summary and other sessionrelated views (paths through the site, entry pages, exit pages, time spent per page).
In table reports, the total row is calculated by summing all other rows. Because visitors cannot be summed in this way, the visitors column in the total row will always be a dash ().