SPOILER ALERT???
It's just an explanation of banding codes that explains my earlier confusion, but If you don't want to risk what might be more of a hint than I think it is, don't read beyond here.
POSSIBLE SPOILER BELOW.
When codes collide, usually all of the involved species take ad hoc codes. But in some cases where one species is rare or has a limited distribution, and the other is commoner or more widely distributed, the ad hoc code may be only used for the less common species. This is illustrated by the last example above, where Barrow's Goldeneye keeps the basic code BAGO even though it could be confused with Barnacle Goose, which gets an ad hoc code.