In an attempt to sum it up in a nutshell:
Taxonomic changes have always been occuring. A taxonomy book is essentially a snapshot of the order of things at the given time of publication. As more information is uncovered concerning known species, new species are discovered, and some species thought to be novel are found to be variants of the same species, the relationships of the various fishes get redone. This can often mean shuffling of certain species to different genera or families, as well as the creation of new ones or simple splitting genera to better represent the interspecific relationships. A lot of the shuffling that has occured recently has been aided by our ability to analyze the DNA of given organisms in order to understand more of the nonsuperficial characteristics and evolutionary connections shared among species. Then, to make things more complicated, some species have been described by more than one person. In that case the latter descriptions are nonapplicable, because the first known properly described name takes precident. This is particularly an issue when a species has a wide range that spreads across the arbitrary divisions of political boundaries.
This is very oversimplified, but it should at least address your question.
