There are a few possibilites
Note that iptables nat rules only work on the first packet of a connection. Once a connection is established the internal mapping tables are used.
Browsers re-use connections to save connection setup overhead. So if your browser still has open connections from before you added the rules requests on those connections will not be diverted.
Another possibility is that you are seeing the affects of caching in the browser. Browsers will often do a "conditional get" which only redownloads the page if the timestamps indicate it has changed since the client last downloaded it.