Next time you decide to comment on a photo that you see on Facebook, make sure, your comment goes on that photo and other comments listed there are in the context of that photo.
An iPhone bug is creating problems to several users on Facebook and you might be one of them. It can happen that when you click the thumbnail of a photo, the photo opening might be different from the photo you saw in thumbnail. The comments appearing under that photo might have been posted in the reference and context of some other photo.
So far this bug has been seen in iPhone and iPad. The only solution that appears to be possible this moment is that you should open the photo album and open the photo from that album.
The users are waiting for Facebook to get rid of this bug problem. It remains to be seen what steps would Facebook take to rectify this problem.