I've noticed that, too. Most of the time, although I don't have any reliable resources to back me up here, just talking from experience, the one that has its fin down is the female. Correct me if I'm wrong. But I've seen this in more than one pair of clowns. I believe that they both have that fin that you're pointing at, but one just have it down most of the time. That's why it appears as if it doesn't have the fin. What Bang Guy said is definitely true the fin also could've been damaged.
Hope it helps a bit...
stupid_naso