the reason it has different AU values is because the business rule column is displaying what the AU's would be for a net nested carrier however as you are serial nested it needs to adjust the AU's to account for the fact that sales in buckets above a class do not affect the availability of that class.
with net nesting the availability calculation is very simple it is just AU - total seats sold in the cabin
but with serial nestng availability = AU - seats sold in or below the bucket selected