I suspect there's more test modes hidden away in some other games; a lot of the games with D552/553 CPUs in them I dumped have inputs that are pulled down, but do not go to anything. It's possible there's extra things present like test modes, or extra skill/options that didn't make it into the finished game.
It's also possible they just pulled down the other inputs for fun, to make PCB routing easier. One thing I found weird is that most bambino games connect say a 3 position skill switch to three inputs on the micro. You only need two inputs to the micro for this. 2 inputs gives 4 combinations, and 3 inputs give 8. I wonder if you input "invalid" combinations if you get more skill levels or something, or test modes or similar.