Just like in Arkham Asylum, it appears that there are different versions of the credits in this game.
Without going into specifics, these are the ones that I know of:
- normal credits (this is the one I got, aka lamesauce)
- Joker singing a song
- Harley singing a song
- transmission from Oracle
Three appear to be random, while one depends on a choice in one of the missions.
Which ending(s) did you get?