Puffing Up Rural Districts With Prisoners

Felons are from cities, but are mostly imprisoned in the countryside. 

The Census does not count them in their home cities, but it does count them in their prison counties.   

Thus, the rural districts inflate their population figures with prisoners, even though the prisoners cannot vote for the district's representatives. 

The felons are subtracted from the population of mostly Democratic districts and used to inflate the populations of mostly Republican districts.

(A second useful point: I learned this interesting fact from a bad book, Hattery and Smith's African American Families Today. We can learn from all sorts of sources, even bad ones.)