I think it's a bug. I saw mine didn't have the AM/PM.. so I did a complete fresh install. Still didn't have the AM/PM so I did the "turn on 24hr... reboot" etc. Had to do it 3 times and the AM/PM finally showed up on the locked screen. Stayed there for about a week and then it went away again. Did the whole "turn on 24hr... reboot" etc. 3-4 times and it eventually came back for a couple days. Now it's gone again. Looks to be a BUG.
Yeah, I now think it's a bug, too. I was showing this AM/PM thing to a coworker and then it just vanished when I selected U.S. Virgin Islands again. Then, fuddling around to see if I could put it back there, there it is.
To put this in my coworker's words, "weirdo"!
Edit: the good thing is that I didn't have to reboot once.