Central America is a region and not a continent...so its subjective as to what some do or dont call "Central America." Its mainly political boundaries with the same culture.
The government of Mexico considers themselves part of North America, yet Google (ie: Google Maps) considers them part of Central America...and corporations, depending upon a certain countries import/export/labor fees, call it whatever works to their advantage.
If you look at maps of Central America, almost all show Mexico included, yet dont list Mexico in the list of countries considered Central America...
Id guess that whoever designates (or originally designated) the taxonomy of cichlids considers cichlids from Mexico to be CA because of the same regional attributes...but Im no scientist so that would just be my assumption.