We pull traffic numbers and statistics via Google Analytics' API (What is an API? Read more here). Google chooses to calculate the numbers that it displays in its interface differently than how it calculates the numbers that it makes available through its API.
For example, if you look at the "Unique Users" metric in your native Google Analytics account, the number that you see does not include duplicate users when reported for an aggregate time period. They strip them out before they report the aggregate number. As a result, you'll notice that the graph that they provide for this metric reports a different number than you would get if you added the unique users reported for each day.
On the other hand, the number that they make available to developers via their API includes duplicate users. It does not strip them out. As a result the graph that we display for the "Unique Users" metric DOES equal the total that you would get if you added up the unique users reported for each day.
So the bottom line is that Google crunch the numbers differently when they report them internally, compared to when they make that same data available to developers.
Although "Users" is used as an example above, this can also apply to other metrics for the same reason.