Why does IE render Arial so differently?

[TruffleShuffle]

The top half is how the page titles look in IE 9.

The bottom half is Chrome.

I can understand a slight difference between browsers but how come IE seems to increase the font size all by itself and Double Bold it?!

here's the CSS:

font-family: Arial, Helvetica, sans-serif;
font-size: 18px;
font-weight:bold;

 

[TruffleShuffle]

It was my colleague who first brought it up so it is like this on at least 2 machines. I'll ask a few more people.