The reason is that Xtreme uses it's own font rendering engine. We do our best to make the overal size of the text similar to the browser but the precise outlook may differ.
As Steve points out, different browsers can also use different rendering engines resulting in slight outlook differences.
As long we talk about officially supported browsers, on Windows platform - IE, FF and Opera use Windows text rendering engine, Safari uses it's own, same as on MacOs. In Safari you can control the level of the font antialiasing thus making text look differently. In Windows you can turn antialiasing off resulting in IE, FF and Opera rendering pixelated fonts.
On other platforms (MacOs, Linux, mobile devices) different text rendering engines are used.
As the result, we can not make it look exactly the same as in all browsers.