I see on http://www.nbcolympics.com/medals :
Why is there 0.5 of a medal?
A recent blog post by Daniel Marcelino seems to indicate what I had suspected. A half of a medal (for statistical purposes) is awarded if an event has a “tie” for the Gold, Silver, or Bronze.
From Daniel’s Blog…
Yep, that’s right; one can get half a medal by tying for a placement.