you would think that as, say (and rounded off) that 90% of home pc users run windows and 10% run linux/mac/unix then this would be reflected in the findings,
With regards to this there remains a few things to take into consideration with this kind of data and how it can be skewed.
Mac: Apple teaches its consumers to rely on Apple for customer support and many of them do which means that most/any cases of mac malware infections would be dealt with in house and Apple is unlikely to publish records of this taking place as it would damage their reputation for 'perfect security'
Linux: People who use linux generally are not your average computer user they have a much greater understanding of how to use and manage their systems and thus would be less likely to need to turn to a forum to get assistance with any problems which do occur for them.
These operating systems (UNIX) are MORE Secure but nothing is perfect.
The increased difficulty and reduced target audience (why target 1 computer when you can get 100?) leads to people not bothering to create malware for these systems more than anything else.