In many political systems, it’s possible to manipulate the likelihood of one party being elected over another by redefining the political districts – include more rural areas in a district to disadvantage the party that’s more popular in cities etc. A similar phenomenon known as the Modifiable Areal Unit Problem (MAUP) can occur when analyzing data. How you define the areas to aggregate your data – e.g. what you define as ‘Northern counties’ – can change the result. The scale used to group data can also have a big impact. Results can vary wildly whether using postcodes, counties or states.
How the new math of Gerrymandering works: The New York Times covers a Gerrymandering case in Wisconsin Understanding the Modifiable Areal Unit Problem [PDF] The Modifiable Areal Unit Problem (MAUP) explained [Wikipedia]