This happens on every sailing of every ship of every major cruise line:

Cruiser A, after checking prices at several or the nation's largest travel agancies and directly with the cruise line finds they all have the exact same fares on offer. But once on board he finds out that the couple with the exact same cabin next door booked with an independent agent who works from home and got the same (or better) fare along with some nice additional perks (pre-paid gratuities, onbard credits, free shore tours, etc. Cruiser A thinks "How is that possible? And how can I find those same deals?"

It's all in the power of groups.

Groups of Cabins

Every seller of cruises reserves blocks of cabins for groups traveling together. They get reduced fares and additional perks and amenities for their group members. They often have extra cabin space they can offer to others looking at that same sailing. The problem is finding the agent/agency who has blocked space on the sailing you want.

Groups of Agents

Independent / Home Based agents normally affiliate with what's called a "Host Agency".

A host agency can often have several thousand independent agents all combining their sales to a point that cruise lines will make sure that the Host Agency and its independent agents have access to the absolute best deals available. That way, a home-based agent can often offer better rates than you'll find directly from the cruise line or from many of the mega online agencies.

Groups of Agencies

Agencies Iincluding host Agencies) will group together into a consortium such as Ensemble, Travel Leaders, Signature, Virtuoso, and others.

Each will have a roster of Preferred Cruise Lines and other suppliers and they won't necesarily overlap. For example, Avalon Waterways is a preferred supplier for Ensemble and Virtuoso but not for Signature. So if you book your river cruise on Avalon with an agent/agency that belongs to Ensemble or Virtuoso you might be eligible for extra benefits that won't be available if you book it through a signature agency. And the reverse might be true for other cruise lnes.

All of these exclusive benefits work their way down to independent / home-based agents through their Host Agency and the consortium it belongs to.