Are you basing this only on Apple's specs? If so I would take them with a grain of salt, until proven otherwise. If you note the Color Classic & Color Classic II have exactly the same VRAM specs. It would not be unusual for Apple to copy and past a profile and change the most obvious major differences between the them but over look something minor. It would have cost Apple a lot more to create two separate boards that were otherwise identical.
Then again, it may have really taken 512K to drive an 8-bit 640x480 over a 512x384 display, since both jump to 16-bit with only 256K (though why doesn't the jump to 16-bit double as well?). If so, I suppose the board was designed for 512K built-in and Apple just put 256K on the CCII, so not a vast difference in manufacturing overhead if that was the only change, just a part substitution.