ClarkMoody is actually grouping it wrong, see for example how goxtool (the reference for MtGox) does it. On both sides it should be summed up from the top of the book down¹ to (and including) the round level, this would then also be the same value you could read from a cumulative depth chart where it is summed up to and including that price. This means the number always represents the amount to buy (or sell) to break this price, its perfectly symmetrical.

¹) Note that on the ask side "down" (deeper into the book) actually means the prices become higher, therefore the inverse sort order and usage of the next higher round price.