Because rose water is subtle, not sugary.

Because it's a suble difference especially if its sugar free. I think they're trying to test your ability to differentiate "perfumed" water vs regular. Sweetened or spiced things are more of a hammer.

As the other commenters said, it's about testing your palate vs. your tongue. I made a mistake in my original comment where I said you don't have a sense of taste without smell, I meant to say you have only taste (sweet, sour, salty, bitter, umami) absent the flavors your olfactory equipment provides.