Free pre-paid DSC is a nice deal!
As for the 10 or 20% off, that is a perfect example of getting different answers from different agents. Since it's for a twelve day and you haven't taken your fall cruise yet, I would guess that's why they are just giving you a percent off offer.
Was you fall comped cruise a certificate from a land based casino? Or was it a direct comp from CAS? If you are looking to see if they will give you another comp, I would suggest you call back after your next cruise. Then they will have a little more info on how you play per cruise. It will probably be easier to get a comp on a 7 day than a 12 day. When you are just beginning with CAS, they will probably only let you have one comped cruise on the books at a time.