I'm sure there's a lot of variables, but in general, why do things like say hemp&emu oil or voltaren seem to work really well for some people and not others?

That is a great question. We are all similar but also different . Many medication, treatments and PT work on some people and not others. There is no single answer to your question.

People respond to different treatments very differently. That is often why there are so many variations of the same product available, because no single treatment is better than others. Have to try to see which one works for you. May be how the medication is metabolized by the individual, or different thresholds of effectiveness.