I've seen the same tests where OSX falls first, of Safari.
But, the reason it appears OSX is more secure is it is quite simply targeted less, as stated already.
Think about it for a second.
If you wanted to plant some advertising malware, and hit as many people as you possibly could,
which web browser or OS would be your primary choice to target?
It would likely be a Windows system, and Internet Explorer, as they are the most commonly used.