Ie8 emulator isn't the same as the real thing

I have been doing a fair bit of work recently for a company do Moodle design.. Many of their staff still use IE8 and XP. Gradually their staff are moving over to IE9 and 10 and Chrome. I have been creating Moodle sites for them and we need to look at the site in all the Browsers including IE8
Now the easiest way of doing this is to fire up a load of browsers on a PC and have a look at the web site on the different browsers. Because of the difficulty in having IE8 and IE9 on the same machine, I have been putting IE10 into emulator mode and running it as IE8. (press F12 to call up the  Code popup) Selecting the Browse Mode as IE8 and the Document Mode as IE8 is supposed to make this browser behave as IE8. Now for most pages this seems to work. Some of Moodle behaves as I would expect it to, but some doesn't. Therefore I have had to either use an old XP machine or to load up the Microsoft emulator and run a copy of XP with IE8. This is a pain, but at least it works.

So when is IE8 not IE8 when IE10 is pretending to be IE8