It may be the difference between internship and residency. As I recall, for ERAS you have to apply to the Army internship program even if your program has a continuous contract. In other words, they may have the surgery residency listed separately from the surgery internship. That's really just a guess though.
I think if you go to the Army GME website, they have the ERAS codes listed for all the Army programs. That should provide a more direct route.