Delta lets passengers choose in advance the option to skip a meal, and save the airline money. Japan Airlines even suggests that doing so is a moral imperative.
This is a trend sweeping the industry, reducing food cost by getting passengers to pre-committing not to eat. United is a part of it too, and they are offering customers more than one choice now to nudge them not to eat. Both of these options that appear when pre-ordering mean the same thing, United can spend less on catering.