It's a weird sentence, but here's what they mean:
The difference between PS and VC is statistically significant.
The difference between PS and PR is statistically significant.
The difference between PS and WM is statistically significant.
Of those three differences, the most common one in the population is found in 17% of the population, and the rarest one is found in 1% of the population. They don't say which one.
We care because sometimes differences are statistically significant (unlikely to occur by chance) but are also quite common within the population. Sometimes those common differences aren't clinically significant -- they're just normal human variation. Sometimes they are common but still clinically significant, because they're reflective of real problems you're having in real life (hence the Jeopardy comment).
