In this alternate world, demigods were human beings which gained divinity from a divine parent. A male deity would knock up a mortal woman, who would carry the child to term. This demigod would share the divine essence of the divine parent, but would be sustained by a mortal womb and fed mortal nutrients. While they would be powerful, they had limitations and were not unkillable. This makes sense in context because half of their makeup would be mortal.
The reverse should be true if the demigod's mother was a goddess. That child would be sustained by a divine essence in a much more direct way due to it literally growing inside a deity. The goddess would also partake in drinking ambrosia, which is a golden liquid that is food for the gods and the key to their immortality. Such resulting beings should be much more powerful and be as immortal as their mother. However, this is not the case. Demigods born to goddesses appear to be just as vulnerable as their mortal counterparts. For example, Achilles, a hero from Greek myth, was born to a goddess and a mortal man, but was bumped off in the most idiotic way by being shot in the foot with an arrow.
I need to justify this contradiction for this world? How could I explain this?