Some agencies have group space for certain sail dates. The price is discounted and there is limited availability. Once their allotment is booked, then the rates will match Disney's for that sail date.
Disney does not allow group rates to be combined with any other offer, including onboard bookings.
As you've just learned, sometimes the savings exceed the onboard offer, including the SBC.
While it would be great to combine both offers, Disney's policies prevent this.