Version parity. It's done all the time when porting to a new platform. Why would you want identical software to have different version numbers on three different platforms.

OK. makes sense. But then how come Win Chrome is already at version 4.

In other words 4 as the major version seems high considering Chrome, for any platform, is fairly new. Compare that with FF which has been for much longer and is at 3.5.x

Are they tracking Webkit or any other version?

No, just trying to compete with IE8 in an enterprise environment. At the speed they're going, they'll catch up pretty soon :)