Its much more random than that - who goes through depends on the order of the public vote and the judges can't predict that unless they know figures and gaps are large and stable.

The rules as are allow the most popular to the final if the most capable is least popular - votes triumph over marks there. The judges have the power to make sure the best dancer reaches the final - they have more power there. They penalise anyone who comes second with the judges and public if the best dancer is unpopular. That all means the vote matters and it means that you don't get the nonsense of a final where the most able isn't there. Unlike the US system neither the votes or the ability is weighed - its just ordered so someone hopeless could make it on one more vote or 50% of the vote regardless of how hopeless they are.

Its messy but its where you end up when the public vote would produce finals without any of the better dancers there. This way you can still get a final outcome where someone poor wins or an unbalanced final, but you at least get one of the best dancers there so you can say the show had something to do with dancing. You just can't do much else when fewer people vote, the vote doesn't necessarily reflect the wider audience and when 90 % of some reality TV votes on similar shows are going to whoever is worst and whoever happens to be male.