Why the huge variation in interventions for the same inputs?

If the same software was giving different outputs for the same inputs in 2 different locations, everyone here would see that as a concern to be addressed.

A software analogy would be: if a client asks for a simple website with a shopping cart and a newsletter, and you ask 10 programmers, you're almost guaranteed to get 10 different platforms (different CMS, different language like PHP vs Ruby vs .net)

It's kind of the same thing with dentists: a good surgeon will tend to lean towards surgery, and there are usually at least 2-3 different ways of solving a particular problem

And is there evidence/theory for "whitening" procedures as preventative care?

Peroxides are potent sterilizers and getting them deeper into teeth may clean deeper than any surface treatments, right?

They prevent you from having yellow teeth.

... I'd say they're reversal agents. They make the yellow part not yellow anymore.