My guess is they don't know the exact origins of the japonica cross which would be a hybrid.
I checked on the RHS in the plant selector which is different to the plant finder, they only have one Astilbe japonica hybrid listed called Astilbe 'Rheinland' (japonica hybrid). Their preferred name is Astilbe 'Rheinland'.
I would think in cases like this that is the way to go, with Japonica Hybrid only in the extra field.
http://apps.rhs.org.uk/plantse...
Searching for Astilbe japonica hybrid in the plant finder, there's several japonica hybrids listed which are or have at some time been offered by nurseries here participating in the RHS scheme.
http://apps.rhs.org.uk/rhsplan...
The first, Astilbe xardensii: Parentage A. chinensis × A. japonica
Astilbe 'Bonn' : Parentage japonica × unknown
Checking some others it's the same story, one says japonica x unknown and the most say nothing which indicates they don't know. Often a good form is found where nature has done the pollinating.
A search for just Astilbe brings up all the records which might be useful.
http://apps.rhs.org.uk/rhsplan...
