The reality is there number of celebs that have zero dance training is fairly low so it will always be about finding a line that feels right. If they only wanted zero then it would be a small pool of people indeed and would be mostly politician, news reader types
90% of actors and singers (and showbiz focused types!) will have been through theater style schools where dance is also learnt to a varying levels. Lots of them will have spent time on the stage etc or grabbing jobs where they can with a bit of dance. Lots of reality stars too
Most people end up with some sort of advantage other the others and a disadvantage in another way
Actors are used to the characterisation and tend to be less self conscious
Musicians are able to keep a beat
Sports people often come in with co ordination and stamina.
Karen might come in with a dance history, but was also part of a team sport where shes less used to individual focus. Amber's been literally plucked from the west end where she's been doing a charltonesque dance, but I'm sure I could dig into most contestants to find a link
Ringers (and people with more experience) carry the first few weeks and it would be dull without them.
I also think that sometimes they start strong but might have taken years to get to that point, often getting eclipsed by the people who are picking it up very quickly and able to progress quicker