I don't think there is anything scientific about the scoring. It's all down to the judges' opinions. A competently executed dance in the first week, using basic moves/steps, will get a favourable reaction from the judges, but by week 10 or 12, the dancers are expected to be using much more complex moves and executing them very well - one could almost say professionally.
As Redhelen says, a couple who dance a perfect waltz and then go on to perform a lousy rhumba probably only do so because they're just not naturally 'in tune' with the rhumba. Whereas a couple who really shine at the rhumba mightn't be able to do a good waltz if their lives depended on it. In theory, the winning couple are the ones who can do both equally well.
I find myself listening more and more to Craig Revel Horwood. He can be quite cruel but he usually speaks the truth, and dancers who take on board what he says often show a marked improvement over the course of the show.