There are a couple other things that come into play.

1. which way the engine is mounted. from the side/from the bottom.
2. how stiff it is mounted. rubber mounted Ie. boat, solid mounted Ie. car

Now what and why the balance has such a significant effect and why it is not set in stone.
The engine as it is reving through its rpm range goes in and out of phase (periods of intense vibration and periods of calm diametrically opposed in the horizontal/vertical axis's) picture a sine wave that crosses two direstions.
why is that significant? because fuel and air vapor has physical mass. Even though this mass is significantly small.
At an rpm level of 30000 rpm the ports are opening 1000 times a second. given their only about 2.5 mm tall you can infer they are only open a few milliseconds at best.
if the vibration of the engine, that can reach levels of 20 G's is creating a shaking force the opposite way the fuel is trying to flow, (at the wrong time)up the port in the engine case it can literally short circuit the fuel transfer literally acting like a governer on the engine. its like you stuck a cork in the carburetor.
Most 12 engines are close enough on balance from the factory and have smaller levels of g force vibration so this is less of a problem. as the engine gets bigger in displacement it can be a significant problem if its not properly addressed.

there you have it
the rest of the real dirt.