It's either:

  - A random number each time
  - Normal split testing using a cookie
  - Split testing using the user agent rather than a cookie 

The last one would, I suppose, be more consistent than using a cookie- after all, most people wouldn't switch browsers, but they might clear their cookies.

That being said, a few companies have gotten in hot water for doing this.

Amazon in 2000: http://news.cnet.com/2100-1017-245631.html

Automattic: http://brianbreslin.com/automattic-caught-ab-testing-pricing... (although they admitted up front they were doing this)

[Edited for readability]