The rice cooker doesn't know either, it will take whatever is needed to take. Short of putting an extensive sensor array and ask you for the kind of rice you put in it, there's probably no way to actually predict it.
On the quick/normal setting, they taste different. My best image would be on black tea: you can use 95C water or near 100C water, one is quicker than the other to fully extract the flavor, but won't result in the same profile (you might actually prefer the quicker one, your choice)
On the UX...welcome to a world where you buy products of different cultures.
If that helps, many people don't actually follow the dosing cup to the letter and adjust the amount of rice and water to their taste, relative the rice we're using at that time after a few tries (at least I do, and so does anyone I talked to), so there is no guideline that would help us.
I'd perhaps compare it to coffee, there are machines doing it all automatically but they taste bland, and the better tools will expect you to know what you're getting into. Rice cookers are as serious business in Japan as espresso machine in Italy, you won't get a high end machine that catters to people who're not familiar with what's going on.