If all the references to objects are not deleted then you won't see it in the garbage because there are still outstanding references.

You might test this by laboriously assigning all references to None and then deleting them, this might get it under control.
This is also very important when using dictionaries in python, where it is the prime source of memory leaks.

I also notice that poser's python is rather greedy about not freeing memory. However, I've never had it get really out of hand.
It hurts me a little because I'm a very lazy programmer and don't always clean up my garbage.