Continuous vs. Peak ratings
The discrepancy you are seeing here is a function of the difference between the way meters and sockets are rated and the way breakers and panels are rated. In particular, meters and meter bases are designated with their continuous ampere rating. A Class 200 meter and meter-base is rated to have 200A run through it 24/7/365, for instance.
This is different from circuit breakers, where a 200A breaker cannot be guaranteed to stay closed if you have it sitting there 24/7/365 with 200A running through it (save for special "100% rated" circuit breakers used in some heavy-duty applications). This is one of the major reasons the NEC has continuous load derates in various places (generally 80% of the marked rating). Furthermore, panel busbar ratings generally work in this fashion, too.
So, you'll see cases where the rating of the metering hardware and the rating of the service-entrance hardware don't appear to match. Smaller sockets will be rated for 100A continuous and used with 100A or 125A services, or 135A continuous to allow usage on 150A services; all of this uses a Class 200 meter as that's the utility's "go to" meter for light-duty work. That Class 200 meter will also be used with a Class 200 socket for some 150A services, as well as 200A and even 225A services. There are even dual-disconnect or tap-lug configurations that allow a single Class 200 meter and socket to serve 250A of load!
Then we get to your case, which is a 400A service using a Class 320 meter and socket, used for any maximum (Article 220 computed) load between 225 and 400A. Larger loads than 320A continuous/400A peak require something other than a self-contained meter and socket though. Some utilities use extended range metering using what's known as a K-Base meter, rated for 400 or 600 continuous amperes, while others simply revert to what's known as current transformer or "CT" metering for all services larger than Class 320, using a box with special transformers in it to step the current down so that a special transformer rated meter can accurately meter it without having the meter in the main current path. Transformer metering requires more work from the utility and more parts, but provides much more flexibility, allowing it to be used for services of any size at any voltage rating.
