Edited to add
Based on the comments on this answer, it appears that Xcode updates via the system Software Update were delayed, appearing quite a bit after they became available for manual download from the developer portal.
It may even be that Apple fits them into times when their network usage was low, which could delay an update even if you clear out your old copy and reinstall the not-quite-up-to-date version. Perhaps the update would appear if you waited several days.
Previous answer
Xcode has never, ever updated for me through Software Update, and it's never updated for the dozen or so developers I work with. The Apple employees that I follow on Twitter -- guys like Jake Behrens, Michael Jurewitz (the dev tools evangelist), Chris Hanson, and Bill Bumgarner -- along with former Apple employees who are devs and other high-profile devs I follow, all tweet something along the lines of, "hey devs, time to visit the developer portal to download the new version of Xcode" when an update comes out.
Are you absolutely certain Xcode updates are arriving automatically for your coworkers? Have you seen it happen? Are you sure they're not pulling your leg? I've certainly never heard of it and consistently hear the opposite, so it sure seems unlikely to me.
The only automatic Xcode updating I'm aware of for Xcode 4, and only then if it was purchased through the Mac App Store and not downloaded from the portal, and only then if you launch the App Store and click on the update, which really isn't very automatic.
Best Answer
I was having this exact same issue all night and wasted a good 3 hours troubleshooting it. I didn't have to go through all of the steps that the other answer provided, but my fix was simply deleting my existing XCode from /Applications using an app called AppZapper (the OS X equivalent of Revo Uninstaller for Windows).
AppZapper makes sure to delete all of the other files associated with the application you are trying to remove. You have to select a checkbox in the preferences in order to be able to delete Apple applications, but as soon as I zapped my existing XCode and emptied the Trash, the new XCode immediately started downloading from the Mac App Store.