A bunch of these cases may just be people finding new details in photos they didn’t notice at first. Or repurposing old photos they had before they joined iNat.
If I’m focusing on a subject, I may miss what’s around it, only to find a second or third organism on review of the photo. It might even be someone else to point it out. Chances are, I’ll only have the one photo to work off of, so I would just have to reuse it for the observation. I’ve got multiple examples of this and it seems that plenty of others do as well. The thread, Observations “hiding” in other observations; Share your examples! comes to mind.