This archived article might shed some light. The most important points seem to be that canonicity is regarded loosely, the total of 81 is reached different ways, some of the books in the official canon were never even printed in Geez, and the Ethiopian Orthodox Church never claimed posession of a complete edition of the Bible in Geez/Amharic. To me this implies that a notion of canonicity is a construct imposed upon their church and not an indigenous aspect. Historically it was probably a more fluid relationship to the status of various texts on the periphery with a tendency towards inclusion/reverence.