Web survey numbers...How do they calculate them?

Web Server Survey

July 2007 => August 2007
http://news.netcraft.com/archives/2007/08/index.html
Apache => 52.65% => 50.92%
Microsoft => 32.84% => 34.28%

http://www.securityspace.com/s_survey/data/200708/index.html
Apache => 73.81% => 73.68%
Microsoft => 19.33% => 19.40%

September 2007 => October 2007
http://news.netcraft.com/archives/2007/10/index.html
Apache => 50.48% => 47.73%
Microsoft => 34.94% => 37.13%

http://www.securityspace.com/s_survey/data/200710/index.html
Apache => 73.62% => 73.53%
Microsoft => 19.40% => 19.31%

Security space mention how they get their results.
http://www.securityspace.com/s_survey/faq.html

I'm not sure how Netcraft gets theirs. The closest thing I found was this:
http://uptime.netcraft.com/up/accuracy.html

How do they get these numbers? What methods do they use? Can they be skewed? How? Its something I'd like to discuss with you folks, because one source says one thing, another source says something else!