Again, my yeoman's Greek is full of misleading notions, but...
Conjecture: The missing root in "αρτον επιουσιον" was from a neologism, perhaps by the first translators to Greek.
"Should we say αρτον eχουσια?"
"Νο, it's not about power or authority, it's about the thing itself. Επι-ουσια is more like it."
"But that's not a word."
"Perhaps you haven't grasped what it is that we're doing here..."