I've been upgraded on DL (while on an Air France ticket and at a time when I had only Silver Medallion status) and couldn't understand why I was given this lucky break -- surely most aboard had bought DL tickets and there were probably others with higher status.

The only reason I was able to think of was that I had asked for a kosher meal. This meant that Delta was able to serve me the same meal no matter where I sat -- and did not have to shell out more money to feed me.