Mike is correct. All the major mass-market cruise lines require everyone to advertise the same cruise for the same price. Anyone doing otherwise risks loosing their credentials. Their prices have to be exactly the same as the cruise line's website, so check there and that will tell you alot. It's possible the information is based on different details and it helps to double check.
Plus, I'll be honest and say I'm not a big fan of those large online booking sites for the reasons CLIA (the Cruise Lines International Association) once stated. They said their investigation found that when booking with online sites and the cruise lines, you find that: 1) the person you're talking to works in a call center; 2) is not a travel agent - they're just an order taker; 3) has worked for the company for less than 6 months; and 4) has never even been on a ship or been to any of the ports, so their information is basically non-existent. Plus, booking with an online site with someone who is not an agent has inherent stress.
I suggest call an agent, preferably a Cruise Specialist, and discuss it with them. They charge nothing for their services and their expertise can be invaluable. Plus, they'll have a great deal of knowledge and personal experience they can share with you that will enhance your vacation and make it more hassle-free.