I was going to tell you that plants within a species can vary widely in their external characteristics (color, habit, etc.), and that taxonomic decisions are made on the basis of DNA analysis, so if the DNA's the same, so are the plants.
After doing some research on Centaurea americana, however, I've found something of a muddle.
Our database does not reflect ITIS, as you said, but the Catalogue of Life. Our information on the Centaurea genus is based on information in the CoL, which cites the June 2014 Global Compositae Checklist as the source of its classification of C. americana as a synonym of C. melitensis.
ITIS and GRIN, on the other hand, say that Centaurea americana is a synonym of Plectocephalus americanus, a name which is not listed in the Catalogue of Life.
The Plant List, as usual, turns things upside down and says that Plectocephalus americanus is a synonym of Centaurea americana. It cites the same source ITIS and GRIN used to declare the opposite, and it also brands Centaurea americana as an "unresolved" name.
The Catalogue of Life is the most recent source, but I wouldn't be surprised if there's some change in the taxonomic status of this name in the near future.
