Regarding the PDF; it's important to realise that the ratio is based just on 2007 statistics. This is a very small sample. I'm not saying it's necessarily misrepresentative, but we can't make any definite claims about what the 5 most competitive courses are, as some people seem to have suggested, just based on it.
And kcfch is right that the quality of applicants varies as well (indeed physicists have the highest number (joint with a few others)).