If Republicans were in charge of drawing district lines, the map could look something like this chart. Despite having only forty percent of the population voting for them, they elect sixty percent of the representatives.

Red wins three to two.

Similarly to the previous scenario, this election outcome is not representative of the population's political preferences.

Cases like this, which are not uncommon in real states, are why some say that redistricting should be undertaken by people (or computers) who do not have a vested interest in the outcome.