From my experience, the "latest update" date on CEAC tracker "Ready" changed a few times for us. I think once was when my fiancée went for the medical exam. I believe the doctor updated their information to the system, and somehow the medical information system is integrated with the CEAC tracker and updates the "latest update" date. Another time could be the consulate updating information on your case. Honestly, I don't know exactly why the "latest update" date changes. But the fact that it gets updated shows progress on your case and that's a good thing.
