Fleet admiral is a military rank, the equivalent of which is used by the service organizations of many civilizations. As a traditional grade, fleet admiral is the most senior flag officer rank of a naval organization, above all the flag admiral grades save for commander-in-chief. In comparison to infantry ranking systems, this rank is equivalent to the most senior general rank.
In Starfleet, fleet admiral exists as a separate rank, but has also been used as a title awarded and referring to officers wearing insignia of vice admiral or admiral, such as Fleet Admiral Shanthi. (TNG: "Redemption II")
In these situations when an officer refers to an admiral as "fleet admiral" they could simply be calling them fleet admiral because they are acting as the head of a fleet. Some have suggested field promotion as another possibility.