I think its about semantics ; with IE/Firefox the choice users make is "install Firefox 3.6" or "install IE 8". Therefore "Firefox 4.0" or "IE 9" are different pieces of software. However with Chrome, the users' choice is "install Chrome" - no version number. Chrome is a software "subscription" rather than a software release.

Maybe these are word games but I think these are important distinctions. And it makes life much easier for web developers: there is only one version of Chrome to design for - the latest one. Occasionally they will back-out/modify a feature and someone will get stung (like I did personally with websocket experimentation) but Chrome moves fast and in the long term there is a single target - a dream compared to the "compatibility mode" approach of IE
http://blogs.msdn.com/b/ie/archive/2010/06/16/ie-s-compatibility-features-for-site-developers.aspx