I think it was originally up to artistic choice. Divergence of how many black rings and how many rays on a particular ring is a slow organic evolution and not because of someone said "our tribe must have so many rings", it just became that way.
I couldn't find the article that I saw awhile ago about which village uses how many rings and rays.
Mata is indeed the ancestor's eyes though. It's called mata no tatara. Tatara refers to a single person or two persons small boat. A 10-person large boat is called chinurikuran. The mata is the most important part of a ship's decoration. Before carving the mata, a special ceremony must held, and once the mata is carved, Tao believes the ship has life.
There are other symbols on a tatara or chinurikuran. There's the man shaped tao-tao wu, tao is the Austronesian root word for man, and tao-tao wu is the "first of men".
There are swirl patterns attached to the tao-tao wu, similar to New Zealand's koru, that symbolizes vitality.
Pairs of black triangles form a column called Ngepengepen, also called the boat's teeth.
The Ngepengepen forms a row of white rhombus, and they are called Osowso.
Finally there's a wave pattern called Volavolow that lines the ship.