This is the reason. Its not a lot, but that is about 10 dungeons just there... Oh, and ignore the Horse Eggs.
I got them from there, too. You forgot that they can also be found in Mineshafts and Jungle/desert temples :3
you said you got them from dungeons dungeons is another word for regular, reoccuring, overworld, generated structures
His point is that there are still thousands and thousands of those on smp9 because the world keeps generating.