SSI Caching

Can someone please explain how the caching of SSI content works on IIS 6? I
have a web site header comprised of an SSI that is periodically updated (once
every few weeks). On more than one occaision, I have noticed that an old copy
of the header is kept in my browser's (IE7) cache for certain pages on the
web site. Shouldn't I see the new header if the SSI has been updated? Or is
caching just dependent on the page itself with no relation to whether the SSI
has been updated?