This is just a theory - one that explains what you have seen. I have no idea if it is correct - but this would make sense if it were the case. I think and believe that you are right. It can still be below the first threshold of a graphic in the directory. I think Dmoz and Yahoo are the two most important criteria on the web for how Google classifies your site.
Page rank gets more press than it deserves - it's just hubs and authorities.
If you do a search on an obscure topic, you may find that the search results are dominated by multiple ODP mirrors. I don't remember noticing them, or for that matter noticing their absence. I have not noticed major mirrors for a while. Most searches only come up with DMOZ and Google, but I have seen some specialized mirrors. Most will have extra text after and before the DMOZ entries.