The web pages on Fluther (and other sites, too) are comprised of many discrete elements. Some of those elements are hosted on different physical web server machines.
For example, I believe that all avatars ultimately source from davinci.fluther.com.
PLEASE RELY ON THE FLUTHER GODS FOR ACTUAL WORKING DETAILS.
Anyways, I think that sometimes, certain servers might get overloaded and not respond to your web browser in a timely manner. The result could be missing avatars. Or the style sheet doesn’t load (meaning the layout of the page will appear whack). Or any number of other variations on this theme.
End result, it’s not necessarily your PC or browser cache. It’s just that flushing your cache is the easiest thing to try for immediate effect. But realize, even if it appears to “solve” the problem, it might simply be that your re-request of the page happens after the server had gotten over the workload hump.