Web browsers (e.g. Netscape and Internet Explorer) typically store frequently visited Web pages on the hard drive of the computer to help speed Web surfing. If a user goes to a site again, and that site hasn't updated, the browser will display much of the content from the local cache. This can result in traffic numbers from a Web site being somewhat under-reported.
To speed surfing, browsers store recently used pages on a user's disk. If a site is revisited, browsers display pages from the disk instead of requesting them from the server. As a result, servers under-count the number of times a page is viewed.
Often a browser will store WebPages in temporary folders on your computer to aid in surfing speed. When someone revisits your webpage and they have a cached version, they will view the old page instead of the new one (there are some metatags and browser settings that will prevent this) leading to your website being undercounted in total page views and visitors.