I also received 2.28.19 "normally" over the air on July 21st. During the month of August, I had stopped briefly at the Sunnyvale service center on the 12th, and for a longer period outside the Tesla store in the Stanford Shopping Center on the 21st, both locations inside geofences marked on the map that's linked in post #3. Neither of those stops triggered any update. However, yesterday (29th), I once again dropped by the Sunnyvale SC, just long enough to pull in and turn around before heading home. 45 minutes later, I received the update notification, and proceeded to get 2.34.100. Days since prior update: 39.
Back on June 13th, over a month before getting 2.28.19, I'd been offered the 2.22.50 update immediately after supercharging at the Monterey/Seaside SC. That was right around the time people were starting to notice a correlation between getting an update notification and having just visited a service center. Days between those two updates: 38.
It's tempting to speculate on what the aging factor might be, but in my case, 30 days wasn't quite enough.