Why do some vending machines have more than one button for water?

Still water.

sparkling water.
and the vending area of that machine.

Water is popular in some places. Since each button refers to a specific area of load in the vending machine, they will sometime fill more than 1 slot with a specific beverage (in this case water) if it's selling quickly so that there is sufficient water for people to buy. It's really just a supply and demand thing.