The only thing I can think of that's more compulsive then asking my dumb question in the first place is ANSWERING IT MYSELF.
Here we go!
It turns out one of the course's facilitators had a hunch that it might have been caused by something her co-facilitator might have pasted into the first posting in the Admin Forum. (Savvy course facilitator!)
So I selected the entire posting and used the Word Doc cleaning tool to whack out all the MS CSS crud.
That did it! Symmetry was restored.
Unfortunately I didn't save the original version somewhere so I could analyze it and see what was in there that caused the formatting glitch that changed the dimensions of the Admin Forum Announcements block.
Guess it'll have to remain a mystery till the next time.
Dr. Bob Blomeyer