It's most likely to do with the way your web browser caches pages... or you may be connected to PPRuNe via a proxy server, which does the same thing for you.
You can try to resolve the problem by changing how Internet Explorer (assuming you're using it) caches pages... check out
Tools/Internet Options/Settings... and change the option to "Every visit to the page".
Note that you may need the help of this thread:
http://www.pprune.org/ubb/NonCGI/For...ML/000993.html if you've got a screwy browser somehow.
Or, to force the browser to grab a single page from the web server, right-click on the page, hold down Control and select Refresh from the pop-up menu.
Hope that helps...