Local website renders differently using (IP address or machine name) vs localhost?
The same css file is used for every page, and F12 in IE8 shows the correct css has been loaded.
Developer Tools should show that IE is not using the same "Browser Mode"/"Document Mode" between the two instances of the site, because that's the problem here. IE defaults to different modes depending on if you're using a machine name or not (amongst other things).
Adding this to your <head> should sort out the problem:
<meta http-equiv="X-UA-Compatible" content="IE=edge">
I had a similar issue, where the sizes differed from the local development site to the one on the production server. Turned out that I forgot the zoom level to 90% when viewing the development version... This answer helped me realize that: https://superuser.com/questions/315448/different-font-size-between-localhost-and-remote-server-in-firefox