Monty,
Different sizes of ACE shafts are designed with various types of carbon fiber to achieve predictable tuning results when going from one size to another. Both the dynamic spine and static spine are accounted for in the ACE designs. Also taken into account is maintenance of a certain mass parameter needed for good downrange performance.
As a result, the 720 utilizes intermediate modulus carbon fiber, while the stiffer sizes use high modulus fiber. The IM fiber is the reason for the mass differential- the shaft ends up with a slight weight increase proportionate to the static spine value.
If the shaft were built with high modulus fiber, it would be too light for the desired performance characteristics. It would also lose some durability.