So uh, what happened in Sweden?
Also, I think it's just a numbers game. It only counts foreign inmates, so any foreign population that is larger than other foreign populations will logically have more inmates.
If there are 1000 Maroccans in Spain, and 200 Tunesians, and 10% of both get incarcerated, it'll still be a larger percentage of Maroccans in prison, even though both nationalities have an incarceration rate of 10%