From the 'Fastest Browser to version 10 Wins!' files:

What up with Google's numbering scheme for Chrome?

The browser is just over a year old and Chrome version 4 was just release last month as a stable product. Instead have a 4.5 version, Chrome is now pushing ahead with version 5 which became available to developers late Friday. Well some developers at least.

Officially the new Chrome dev-channel release is numbered Chrome 5.0.307.1 and it was first made available to Windows and Mac users (no Linux release in tandem this time).

As to what's new in Chrome 5 that merits this new number, that's not an easy question to answer. Google's release notes are very sparse on the release and even though I'm running it myself now on a Windows test box (while waiting for my Linux version), there are few user-visible changes.

As always there are bug fixes for stability including a key one for the Mac version for a browser crash when deleting cookies. The big key item that Chrome 5 does have is the beginning of a new content privacy UI.

No it's not something that in my opinion, any other modern browser would say warrants a new number for a browser, but hey I don't walk the halls of the Googleplex so perhaps there is a good reason. Maybe Google is trying to release Chrome 9 before Microsoft releases IE 9? At this rate it'll be no contest.