Actually, additive is not entirely correct. Values are not additive. If a later preset changes a value that has already been set, that new value will replace the old one. For example, preset 1 has white set to +30, but preset 2 (applied later) has white set to +10, the new value will be +10. But anything not changed in a later preset will remain. So preset 1 might have black set to -20 and preset 2 doesn't change black, the result will still be -20.