I suggest you email the RHS with that question - it is a perfectly fair one. Meanwhile, here's my explanation and thoughts...

If you click on the word 'Viola' at the top of the listing, it gives the explanation of the codes in brackets, viz:
(C) Cornuta Hybrid
(dVt) Double Violet
(ExVa) Exhibition Viola
(FP) Fancy Pansy
(PVt) Parma Violet
(SP) Show Pansy
(T) Tricolor
(Va) Viola
(Vt) Violet
(Vtta) Violetta
[http://www.rhs.org.uk/rhsplantfinder/GenusClass.asp?Genus=Viola]

These constitute an extra tier of classification, of more use horticulturally than botanically. V. x wittrockiana and V. x williamsii are similar, though more botanical, categories. Within all these categories, there are cultivars, and for brevity it is these cultivars that the Plant Finder concentrates on.

It can be argued, though, V. x wittrockiana and V. x williamsii should be included; these are names which people may come across in books and want to find suppliers for - that being the principal aim of the Plant Finder. And it is perhaps with the suppliers that some 'blame' has to be laid for the omission, since the Plant Finder lists what suppliers have for sale (with some modification for the sake of consistency of terminology across the wider world of garden plants). How many suppliers list V. x wittrockiana or V. x williamsii just like that - without specifiying a cultivar name?... Very few, I would think.

In practical terms, therefore, when using the Plant Finder, I think one needs to gain awareness that:
- V. x wittrockiana = Fancy Pansy (FP) and Show Pansy (SP)
- V. x williamsii = Viola (Va), Violetta (Vtta), Exhibition Viola (ExVa) and some of the cornuta hybrids (C)
(no doubt somebody will correct me if I am wrong in that simplification)

By the way, you could also come across V. x stuartii. That name was proposed in 1939 by Bergmans. But Wittrock coined V. x williamsii in 1897, and so V. x stuartii was correctly abandoned.

But talking of abandonment, it is rare to come across mention of V. x williamsii these days. That is partly because Wittrock was not the only person to define the name (the RHS qualifies the name as V. x williamsii auctorum), and partly because, due to sheer complexity of the hybrid origins of some of the cultivars, it was difficult to be certain whether they should be pigeon-holed there or not - rendering the pigeon-hole somewhat useless.

V. x wittrockiana has survived because its scope is more limited and it has only ever been defined once.

Hope that helps a little.