I've noticed that my fish get them sometimes where fin damage has grown back. I've often wondered if it was sort of like scar tissue. It doesn't seem to affect them in any adverse way and some of my fish have had it for 6+ years.
In all honesty you could probably trim off the fin and it'd grow back properly without it, but that's a lot of work because then you have to keep the water really clean so that it grows back properly. In my case, I just leave it. It's a cosmetic thing and it does irk me sometimes but I guess as long as the fish are healthy and spawning that's all that matters.