I am also not sure of the reasons behind this, but I think it has to do with the sorting and extraneous entries in the database.
I do need to get to the bottom of it, but the sure fix for now is to go into the administration page and click "Refresh" under the stats section on the front page. This does a full garbage collection on the database and makes sure everything it in sync, and fixes this problem.
Like I said, I will look for the reason why this is happening -- even if there are extras in the database it shouldn't affect image navigation like this.