Their point is that they are not a store, you are focusing on the key-value aspect. In a store, if I write, the expectation is that the write will occur. They g

Their point is that they are not a store, you are focusing on the key-value aspect. In a store, if I write, the expectation is that the write will occur. They gain efficiency by sometimes deciding the write is not worth the cost and evicting rather than writing.