You should check the quotes very carefully. Something is different between them.
The Celebrity pricing policy doesnot hold to the brochure rate. Celebrity itself will sell you a cruise that is discounted well below that price. The policy actually is that whatever the price being charged by Celebrity itself is exactly the price that all TAs will also charge. None of them have the authority to vary on the cruise itself.
So, while you think the quotes are different, it is probably due to the fact that the various agencies are stating it in different ways. For example, perhaps one isn't including the taxes (which you will most assuredly pay). Perhaps one is not including the port charges. If they were exactly "apples to apples" you would get exactly the same price for the cruise itself.
Their are other add-ons -- like air travel to the port city and home, cruise insurance, hotel add-ons, land excursions, etc -- which are not covered by the Celebrity pricing policy. So, different TA's can price them (or give them away) any way they want. So, if your price comparisons included any of these, that is where the price difference is.
If you have actually found a TA that is pricing cruises lower than Celebrity, we'd all like to know who it is!
Ted