Question:
When I view the Monthly Visitors report, the values for unique visitors do not equal the summed up values from the Daily Visitors report.  Shouldn't these columns add up to the same value?

Answer:
The best way to explain why the daily unique visitors report does not add up to the monthly unique visitors report is with a story unrelated to web site statistics at all.

Let's assume that you own a book store, and you only have one customer.  Let us then assume that this customer visits you on the following days:

Mon, Mar 9, 2009
Wed, Mar 11, 2009

If you made a report that explained how many individual people came to your store by day, the report would look like this:

Mon, Mar 9, 2009 - 1 person
Tue, Mar 10, 2009 - 0
Wed, Mar 11, 2009 - 1 person
Thu, Mar 12, 2009 - 0
Fri, Mar 13, 2009 - 0
etc.

Now lets assume you made a report showing the number of individual people that came to your store each month, it would look like this:

Feb, 2009 - 0
Mar, 2009 - 1 person
Apr, 2009 - 0

You only had one customer, so no matter how many times he visited your store in March, your report will always show 1, because you only had one person in your store.

These two reports are showing the same type of information, individual people in your store during a given time period.  They show different numbers, but they are both correct.

Unique Visitors in SmarterStats function in the same manner as individuals in a store.  Different time frames on the report will yield different values, because the number is showing how many unique visitors there were to your site during the time frame on that row.

It is for this reason that the columns in the report are not totalled.  The two reports, though named similarly, represent very different data and cannot be compared.