The addition of digits is known as "theosophical reduction," and is a common way of rendering a set of numbers into a single card. There are a couple of other ways: "casting out nines" (or removing increments of 9) until you get below 22, and simply subtracting 22 to the same effect. In the first case, 23 will reduce to either 14 or 5, depending on which one best fits the reading, and in the second case 23 reduces to 1.