First in time takes priority
, the reason the name is undetermined we don't know. It could be a simple answer such as nobody has got around to doing the determination.
Centaurea americana is the earliest named, and it has more references on Tropicos as well as images. The IPNI only lists this, there is no reference on the IPNI for Plectocephalus americanus.
The Plant List is a conglomeration of scientific resources, it has changed the reference to Plectocephalus americanus ....
This name was in version 1 of The Plant List, record gcc-14994.
•this name record has changed
•names related to this name have changed
I would take the record from the IPNI as the correct one, look at the source references of both:
Plectocephalus americanus ..
Published In: The British Flower Garden, . . . series 2 1: t. 51. 1830. (Jun 1830) (Brit. Fl. Gard., ser. 2)
Centaurea americana ...
Published In: Journal of the Academy of Natural Sciences of Philadelphia 2(1): 117–118. 1821.
The indecisiveness could be that DNA tests are required to determine if they are actually the same plant, taxonomists are now renaming many plants due to DNA where originally the only thing they had to go on was physical structure.
I see no reason why Centaurea americana can't be listed.