The website states these are the numbers of licensed bikes, i.e. those that have current road tax. It does say that the figures are as accurate as they can get. I did notice looking at my registration document that my Rivale is shown as an S3 under model with no mention anywhere of the Rivale name so I guess my bike isn't included in that tally. It's probably more useful to compare the relative numbers between different models - easy to spot which are the most popular.
2005 Aprilia RSV Factory