Not all visitors are tracked by Google Analytics, that depends on many factors that are required by Google that may not always be satisfied by the visitor.
Different web analytics products may use a variety of methods to track visits to your website. Therefore, it is normal to see discrepancies between reports created by various products. However, we generally believe that the best way to think of metrics across different web analytics programs is think in terms of trends, as opposed to numbers by themselves.
One example is to compare related metrics, such as pageviews (eg. 15% of traffic went to page x). In addition, the comparison of data over time could be valuable - information such as "conversions increased by 20% over the past 3 months," or "our site gained 10% more pageviews in the month of March." In most cases, you'll notice that different analytics solutions, though different in numbers, will generally depict the same trends.
Our counter will count a visit every time a visitors is sent to your site. We strongly suggest that you use "server-side" counters, which work with your site's Raw Access Logs which count your real visits.