After some more searching, I have managed to find two instances of the same problem occurring to other people. I don't believe this is an issue with the CSS anymore but I'm not sure whether the issue is due to Google's actual font or how Firefox and Chrome decides to render this particular font.
Here are the two links.
http://code.google.com/p/googlefontdirectory/issues/detail?id=37 http://productforums.google.com/forum/#!topic/chrome/QofmpbyZ7sQ
My solution which was taken from the first link was to download the bold non-italic font from FontSquirrel and embed it into my site.