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
It seems to me the main goal here is that you need to regularly update multiple Macs and you'd like to do that without having to download them each time for each machine.
I see two potential options here:
Content caching
According to Apple:
Source: Apple - About content caching on Mac
In essence, you can enable Content Caching via the Sharing preferences pane in System Preferences to manage content caching on your Mac and select the items you want cached. Once cached, other Macs on your network can use this to perform updates from.
Refer to About content caching on Mac and Manage content caching on Mac on Apple's website for more info.
Download updates manually
You can download all updates directly from Apple by pointing your browser to: https://support.apple.com/downloads
From there you'll see the most recent software updates available for download, and you can also use the buttons at top to browse downloads by product. There's also a search field you can use to conduct specific queries.
Downloads will be in the
.dmg
disk image format and you can then keep and use these as required on your multiple Macs.