Maybe the US Dept of Agriculture hasn't updated their info lately, that takes a lot of work and most places don't have the time, resources nor the desire to do this.
For each plant type there is a taxonomic holder, you can find a list on ISIS. It is each registrars responsibility to maintain correct records for their given plant genera. These are spread worldwide..
http://www.ishs.org/sci/taxlis...
You will find the RHS is listed here as holding the International taxonomic list..
http://www.ishs.org/sci/taxlis...
