Sunny 16 teaches you the difference between full sun and hazy. Hazy and cloudy. Cloudy and overcast. I'm going to bet it wasn't really full sun. What did the shadows look like?

On the point about the ASA change. Download this:

The exposure suggestions are Sunny 16. Last I checked Japan is pretty far north to. I don't know if Ilford still provides little inserts with exposure suggestions but if they do I bet it's Sunny 16 to. last I checked Agfa used Sunny 16 in it's box inserts.

Sunny 16 isn't intended to be used with the box speed but with the film speed that works best with your system. If you wouldn't set the box speed on an exposure meter then why would you use it with Sunny 16?