I answered it - this particular butterfly bush is a hybrid. Hence Botanical designation: Buddleja crispa×Buddleja marubiafolia from the Google page I linked to. Now, if a simple hybrid parentage like this gets its own name published for it at some point - as was done with Prunus x yedoensis - then named horticultural selections from the same cross may get repeatedly listed with the hybrid species name included. When authors are aware of it. As in P. x yedoensis 'Akebono'. Otherwise if hybrid parentages are not known or do not have their own names - as when involved genetic backgrounds are too complex for this approach - then it is routine to just give the genus and cultivar. For an example of an apparent result of one of these latter situations we have to look no farther than the P. 'Shirotae' above your name.