This is a key distinction. Plus codes map to a 14m x 14m square area and aren't aware of things like building units or elevation.

If you live in a high rise apartment, a plus code does not identify you precisely. Sadly to do this you need some knowledge of a structure's internals. It makes sense it's being done on the national level in Japan.

The default +2 plus code is ~14m resolution. But you can add more characters to the end to get higher res. 5 more gets you down to sub centimeters.

Or even just addresses that are split between multiple pluscodes?

For example, a duplex where the front door of each unit is adjacent to the other. Even at the 4m resolution, that means both units front doors (and thus street addresses) can fall into one single pluscode.