Site layout differs between using http://hostname and http://ip in the same browser

I had no idea where to put this, so if it belongs somewhere else, just give me a heads up.

I'm just in the middle of updating my webserver and debugging a site for ie, and I noticed something weird:
When I go onto the site using the normal url (http://hostname) it looks different than when I use the IP address in the browser (http://xx.xx.x.xx).

Does anyone know why this is? It only seems to happen in IE.

Screenshots attached:
1. Normal url.
2. Direct IP.

• Notice the (superfish) menu is no longer 100% width in no.2.
• The table in the calendar appears as I wanted it to in no.2 (every other line is pink), but doesn't in no.1.
• Some fonts (weights) appear different & the 'log in' button is a different shape.
• Spacing and margins are different. No.1 overflows the page width, but no.2 doesn't.

Can anyone shed any light on this? It's not a major issue, it'd just be good to understand!