Web servers aren't required to send a 304 header as the response when you send an 'If-Modified-Since' header. They're free to send a HTTP 200 and send the entire page again.
Sending a 'If-Modified-Since' or 'If-None-Since' alerts the server that you'd like a cached response if available. It's like sending an 'Accept-Encoding: gzip, deflate' header -- you're just telling the server you'll accept something, not requiring it.