UCLB3 is from Samsung. It's just as "official" as that UCKL2 GB that people were downloading through Kies.
Because neither were truly official. Like I keep on saying, that UCKL2 was never meant to be the official update. The dev team put the brakes on it last minute so Samsung/AT&T never announced it, but for some odd reason, one of the AT&T reps posted on a forum that it was available.
The Samsung dev team made it so that if you're running the latest Froyo software (which everyone should have gotten updated via over-the-air back last year), which is UCKH1, Kies wouldn't work. They didn't realize that not everyone updated their phones OTA and were still running an outdated version of Froyo. These people were able to get "updated" to GB UCKL2, which again, was never officially announced.
Again, like I said, UCKL2 had a few bugs. Some people were having totally screwed up text messaging issues, there were screen sensitivity issues, and more.
UCLB3 is currently the latest leak by Samsung. UCKL2 (what people keep on referring to as the "official", even though it isn't) was also nothing more than a leak. It just happened to be accidentally be available via Kies for the outdated Froyo versions.
UCLB3, being the latest build, fixes a few bugs from UCKL2 and is thought by many as currently being the best version of GB on this phone. If you want it, you need to use this:
[AT&T] [ROM] [UCLB3] Heimdall One-Click Return to Stock + Easy CWM Installation - xda-developers
It doesn't require rooting. It's a similar concept to ODIN, except via a different program called Heimdall. You're essentially taking the phone back to "factory settings", albeit on a GB build. It is flashing though, so there is always a possibility that you might not have followed a step or have done something to cause it to mess up. That's your responsibility. However, I will guarantee that if you follow all the steps down to the T, you will 99% not have any issues.
Hope that clears it up.