Web browsers have default settings aimed at people with slow connections or expensive bandwidth charges. When you visit a site a copy is stored on the PC and if you visit again in the following few days it saves time and bandwidth by showing the saved (cached) copy instead of going back over the web.

How you change the defaults depends on what browser you are using but if you think you are seeing an old version of a page just try pressing CTRL + F5 a few times. That tells the browser to go back over the web and get the absolute latest version.