Maybe Because It's A Double Planet
Charon doesn't orbit Pluto. It orbits a system center of gravity outside of Pluto. I think the same is true of Pluto in that the wobble effect induced by Charon is greater than Pluto's radius. Just an educated guess. Charon is bigger than Ceres. So if you use the size and roundness criterion with the caveat that it doesn't orbit a planet it kind of makes sense. Unexpected, but scientifically accurate IMO.