The seven homunculus that have been named so far represents the seven deadly sins, given father's attitudes about humans (they're insects to be judged only on their usefulness), it's logical that he would get rid of anything tying him to humanity, including these human sins. But if he expelled these sins, why would he be the emotionless monster that he is today? I think the most likely scenario is that he went even further and expelled from himself all remnants of his humanity in addition to the seven sins, and he put this in the form of another homunculus, and what else would you name the humanity that you're trying to rid yourself of, aside from your own human name?
This would explain father's situation (semi-human being without a name or any apparent emotion), and Hohenheim's position as a homunculus that can father children; due to his nature, he's simply too human, he isn't represented by any concept like greed, envy, or lust, just a desire to live a human life.
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