What is the difference between still water and tap water ?Feel free to just provide example sentences.

I have been living in New York for three years. As you may know, tap water here is drinkable, although I think many people still use purifying filters. At restaurants, waiters often ask: "Would you like tap or sparkling water?" or "Still or sparkling water?" I thought tap water and still water were completely interchangeable in this context (both are contrasted with sparkling water). However, yesterday a waiter asked me: "Would you like sparkling, still or tap water?" What do you think is the difference between still and tap water in this phrase? I answered "tap water", and other customers next to my table also answered "tap water". Don't you think the choice "still water" here is unnecessary? Thank you!