As many have stated before it is quite hard to calculate the actual Linux market share.
We remain hidden from the traditional statistics grabbing mechanisms thanks to the following:
[li]The lack of forced registration for Operating Systems installations[/li]
[li]The ability to mask our OS and Browser on web servers, typically done to allow us to access windows centered websites.[/li]
[li]The effectiveness of our firewalls, blocking OS discovery scans[/li]
[li]The clean network traffic, without the plethora of announcement packets that other OSs send you cannot just sniff to find the Linux boxes.[/li]
I believe that if you count the server and desktop installations that Linux has far more than a 5% market share, however the freedoms we enjoy as Free Software users and highly customizable applications we use are the reasons that we cannot confirm any numbers as accurate.
In particular the OS masking through browsers is what makes the report your are citing inaccurate.