What's much more likely to be happening is that the web server is serving out different versions of the same page dependent on the browser's 'user-agent' setting. You can verify this by changing the user-agent setting to whatever it is that Firefox uses by default. Something like this should allow you to change the setting for IE.

Here's a pretty comprehensive list of existing user-agent strings, should you want IE to masquerade as anything else.

Hope that helps