This is just the different ways that browsers handle text and IE9 uses "sub pixel positioning" which can reduce the line length slightly.
It depends on font as to how different things are but you get the same effect with browsers on mac systems and lines of text can be up to 25px different in total length (even if you control letter spacing). I often see menus wrapping to another line when the author expected them to fit on one line.
You just have to allow that text on browsers will be different even before the user has applied their own settings and dpi settings. You have little control over this. Always allow breathing space and code accordingly.
Text at 10px is really too small to read for most people anyway but that's another issue :)
There are so many differences between two browsers, ie displays the characters in a way different than all other browsers. Even every browsers have their own set of style and characters. In firefox you will not find the text size but in ie you will find that too.