contract fuel
Here is the deal on contract fuel. They go to the FBO and say they can bring it business and agree to an into plane price structure, usually based on the various volume cutoffs. The margin stays the same but the cost of fuel changes regularly therefore price changes that you see may relate to the day you look and whether or not that particular company changes price on what particular day.
Every once in a while, Colt or someone else may run special prices --by time or for special customers.
Most airline sized equipment use Mariah, or other contract fuel companies. World would fall under both as they do so much arranging on foreign trips. Sometimes you can make just as good of deal dealing directly with the FBO, expecially if you have a fuel hog.
At the major fuel companies, they have entities that deal with the FBO's but also commercial departments for huge accounts. They may have an entirely different deal as well.
Check around, there is no all the time way of being the lowest.