This may be caused by your browser's cache settings. The 'cache' feature of web browsers was developed as a way to speed Internet access at a time when most users were using slow dial-up access. When a page is loaded, your browser places the page in its 'cache', and, rather than reload the page from the Internet, it reads the page from its cache files on the hard drive. If the browser could read the files from the hard drive rather than wait for a download each time a page was asked for, it would appear much faster.

In, for example, Safari's Safari menu you can choose Empty Cache to delete all cache files. This will force Safari to reload web pages that it has already downloaded. Other browsers have a similar menu command. You can also reload a page in any browser by holding down the Shift key before the page loads.